首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   6025篇
  免费   215篇
  国内免费   20篇
化学   4001篇
晶体学   102篇
力学   160篇
数学   706篇
物理学   1291篇
  2023年   38篇
  2022年   50篇
  2021年   65篇
  2020年   118篇
  2019年   107篇
  2018年   62篇
  2017年   63篇
  2016年   155篇
  2015年   168篇
  2014年   144篇
  2013年   230篇
  2012年   268篇
  2011年   329篇
  2010年   208篇
  2009年   186篇
  2008年   271篇
  2007年   259篇
  2006年   256篇
  2005年   267篇
  2004年   225篇
  2003年   191篇
  2002年   211篇
  2001年   122篇
  2000年   117篇
  1999年   92篇
  1998年   84篇
  1997年   101篇
  1996年   95篇
  1995年   86篇
  1994年   80篇
  1993年   77篇
  1992年   69篇
  1991年   58篇
  1990年   56篇
  1989年   57篇
  1988年   59篇
  1987年   65篇
  1986年   56篇
  1985年   47篇
  1984年   44篇
  1983年   53篇
  1982年   41篇
  1981年   59篇
  1980年   45篇
  1979年   57篇
  1978年   61篇
  1977年   50篇
  1976年   46篇
  1974年   37篇
  1973年   39篇
排序方式: 共有6260条查询结果,搜索用时 0 毫秒
11.
C. D. Wagner 《Tetrahedron》1961,14(3-4):164-174
Radiolysis of propylene gives mainly hydrogen, and dimeric, trimeric, and other low molecular weight polymeric hydrocarbons.

Detailed analysis of the dimer shows the products to be, in order of concentration, 4-methyl-1-pentene, 1,5-hexadiene, 1-hexene, 2-methylpentane, 2,3-dimethylbutane, 4-methyl-2-pentene, 2-methyl-1-pentene, 2-hexene, and n-hexane.

The relative product concentrations, and the isotope species distribution in the products obtained from radiolysis of a 50:50 mixture of propylene and propylene-d6, demonstrate that the alkanes, the diene, and much of the olefinic products are formed by combinations of n-propyl, isopropyl, and allyl radicals.

Isotopic species distributions in 4-methyl-1-pentene, 1-hexene, and 2-hexene demonstrate that appreciable fractions of each of these products are formed by a direct condensation of two propylene molecules with intramolecular hydrogen rearrangement. The previously postulated direct dimerization is thus verified, and the idea of its being an ion-molecule condensation receives further support.  相似文献   

12.
13.
14.
Numerical estimates of the Kolmogorov-Sinai entropy based on a finite amount of data decay towards zero in the relevant limits. Rewriting differences of block entropies as averages over decay rates, and ignoring all parts of the sample where these rates are uncomputable because of the lack of neighbours, yields improved entropy estimates. In the same way, the scaling range for estimates of the information dimension can be extended considerably. The improvement is demonstrated for experimental data. (c) 1996 American Institute of Physics.  相似文献   
15.
On noise reduction methods for chaotic data   总被引:1,自引:0,他引:1  
Recently proposed noise reduction methods for nonlinear chaotic time sequences with additive noise are analyzed and generalized. All these methods have in common that they work iteratively, and that in each step of the iteration the noise is suppressed by requiring locally linear relations among the delay coordinates, i.e., by moving the delay vectors towards some smooth manifold. The different methods can be compared unambiguously in the case of strictly hyperbolic systems corrupted by measurement noise of infinitesimally low level. It was found that all proposed methods converge in this ideal case, but not equally fast. Different problems arise if the system is not hyperbolic, and at higher noise levels. A new scheme which seems to avoid most of these problems is proposed and tested, and seems to give the best noise reduction so far. Moreover, large improvements are possible within the new scheme and the previous schemes if their parameters are not kept fixed during the iteration, and if corrections are included which take into account the curvature of the attracting manifold. Finally, the fact that comparison with simple low-pass filters tends to overestimate the relative achievements of these nonlinear noise reduction schemes is stressed, and it is suggested that they should be compared to Wiener-type filters.  相似文献   
16.
17.
18.
The isotopic exchange of35S between thiosulfate/sulfite and tetrathionate/thiosulfate is examined. The kinetic data are determined. In both cases the reactions are of second order. As the isotopic exchange between tetrathionate/thiosulfate is very fast, these reaction components are used in high dilution. Furthermore, the extent of exchange during the separation is determined.   相似文献   
19.
Abstract The short-term adaptation of intact leaves to an increase in light intensity was studied by an analysis of chlorophyll fluorescence and oxygen evolution monitored by photoacoustics. An increase in light intensity led to an oxygen “gush”. This “gush” was followed by a large (up to 120%) biphasic increase in the yield of oxygen evolution characterized by a fast phase (T = 0.5–2 min) and a slow phase (T = 4–20 min). The fast phase of the increase in oxygen yield was coupled to a decrease of fluorescence, whereas the slow phase was accompanied by a parallel fluorescence increase. A comparison of fluorescence parameters with oxygen yield indicates that the slow phase of the increase in oxygen yield was coupled to an increase in the antenna size of photosystem II. The slow phase was not inhibited by the uncoupler Nigericin but it was absent in chlorophyll-b-less barley mutants dencient in the light harvesting chlorophyll a/b protein complex of photosystem II (LHC II). These experiments indicate that changes in the LHC II mediated energy distribution, which occur in the time-range of several minutes, are involved in the adaptation to changing light intensities. Moreover, electrophoretic analysis of 32P orthophosphate labeled leaf discs adapted to low and high light intensities suggests that the slow phase of the increase in oxygen evolution involves dephosphorylation of the 25 kDa polypeptide of LHC II, by a small extent of 12%. The trigger for the slow phase of the increase in oxygen yield does not involve the oxidation of the plastoquinone pool. It was found that in response to the increased light intensity, the plastoquinone pool became more reduced as judged by model calculations. Experiments with the uncoupler Nigericin suggest that the control of the slow phase of adaptation to increased light intensity was also not exerted by the pH gradient across the thylakoid membrane. The similarities between the adaptation to increased light intensity and the state II to state I transition suggest that both adaptation phenomena involve LHC II dephosphorylation possibly triggered by the cytochrome b6/f complex.  相似文献   
20.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号