全文获取类型
收费全文 | 188186篇 |
免费 | 2831篇 |
国内免费 | 512篇 |
专业分类
化学 | 101184篇 |
晶体学 | 2220篇 |
力学 | 7640篇 |
综合类 | 12篇 |
数学 | 21594篇 |
物理学 | 58879篇 |
出版年
2020年 | 1577篇 |
2019年 | 1541篇 |
2018年 | 1952篇 |
2017年 | 1931篇 |
2016年 | 3450篇 |
2015年 | 2308篇 |
2014年 | 3310篇 |
2013年 | 8057篇 |
2012年 | 6568篇 |
2011年 | 8044篇 |
2010年 | 5481篇 |
2009年 | 5282篇 |
2008年 | 7333篇 |
2007年 | 7313篇 |
2006年 | 6875篇 |
2005年 | 6301篇 |
2004年 | 5544篇 |
2003年 | 4792篇 |
2002年 | 4701篇 |
2001年 | 5064篇 |
2000年 | 3925篇 |
1999年 | 3018篇 |
1998年 | 2620篇 |
1997年 | 2563篇 |
1996年 | 2450篇 |
1995年 | 2337篇 |
1994年 | 2362篇 |
1993年 | 2193篇 |
1992年 | 2519篇 |
1991年 | 2459篇 |
1990年 | 2269篇 |
1989年 | 2240篇 |
1988年 | 2157篇 |
1987年 | 2077篇 |
1986年 | 1972篇 |
1985年 | 2649篇 |
1984年 | 2758篇 |
1983年 | 2261篇 |
1982年 | 2462篇 |
1981年 | 2334篇 |
1980年 | 2300篇 |
1979年 | 2413篇 |
1978年 | 2559篇 |
1977年 | 2443篇 |
1976年 | 2445篇 |
1975年 | 2357篇 |
1974年 | 2405篇 |
1973年 | 2346篇 |
1972年 | 1516篇 |
1971年 | 1370篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Polymer lightguides of PMMA doped with the azo dyes methyl-red and N, N-dihexyl-4-amino-4'-nitro-azobenzene (DHANA) are investigated with respect to their opto-optical properties. The trans-cis photoisomerisation of the dyes leads to reversible polarization-dependent optically induced refractive index changes. The index changes are anisotropic. Birefringence is measured using index matched lightguides. The changes with respect to each polarization are determined by leaky mode spectroscopy and by using a half-integrated interferometer. These changes are intensity dependent and, with a suitable geometry, bistability at low intensities (3 mW/mm3) can be obtained. 相似文献
992.
Three-dimensional astigmatic resonators, typical examples of which are the resonators bounded by cylindrical-spherical mirrors and cylindrical-cylindrical mirrors oriented at an arbitrary crossed angle, are investigated in detail by using a complex curvature tensor concept and generalized tensor ABCD law. Computerized numerical calculations illustrate some interesting characteristics of these astigmatic resonators. 相似文献
993.
Summary Oblique propagating magnetohydrodynamic waves with various wave forms and amplitudes are observed both at the Earth's foreshock
and at comets. The possibility of interpreting some observational results in terms of nonlinear evolution of one- and two-dimensional
hydromagnetic waves is investigated. For this purpose both analytical and numerical techniques are employed. It is found that
an initial monochromatic wave changes its polarization giving origin to magnetosonic shocks and rotational discontinuities;
the time evolution of density-magnetic-field correlation is studied, as a function of the plasma parameters and of the propagation
angle. In the two-dimensional case both a transverse instability and a self-focusing effect may take place. Moreover, a two-dimensional
magnetosonic solution is found, in which the density fluctuations are driven by the total pressure fluctuation as in a one-dimensional
simple wave. These theoretical predictions compare well with the features observed in the solar-wind waves.
Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990. 相似文献
994.
995.
T Christensen E B Jensen F Junker H Dalb?ge F Abildgaard J J Led 《Acta chemica Scandinavica (Copenhagen, Denmark : 1989)》1992,46(1):97-99
Biosynthetic human growth hormone specifically 13C-labelled in the carbonyl positions of all 26 leucine residues has been obtained by recombinant DNA techniques using 13C-labelled leucine and an E. coli strain that requires leucine. It is shown that, on the whole, the labelling is specific with no significant mislabelling as would have been the case had the 13C-labelled leucine been metabolized. 相似文献
996.
997.
Plasmatic accumulation of uraemic toxins in the middle molecular mass range has been reported to be associated with several pathologies observed in uraemic patients. The very low concentration of these toxins in uraemic body fluids makes classical chromatography techniques inadequate in isolating sufficient amounts of these endogenous substances, thus precluding their identification. A scaling up of gel permeation and ion-exchange chromatographies was therefore developed. This considerably increased the amount of uraemic toxins isolated, thus allowing the study of their chemical nature and facilitating understanding of their biological activities. 相似文献
998.
B. Blank J. -J. Gaimard H. Geissel K. -H. Schmidt H. Stelzer K. Sümmerer D. Bazin R. Del Moral J. P. Dufour A. Fleury F. Hubert H. -G. Clerc M. Steiner 《Zeitschrift für Physik A Hadrons and Nuclei》1992,343(4):375-379
Total charge-changing cross sections have been measured for8Li on C and Pb targets, for9Li on C, Al, Cu, Sn and Pb targets, as well as for11Li on C, Sn and Pb targets at about 80 MeV/nucleon. These data are compared to measured total reaction cross sections and Glauber-type calculations using Hartree-Fock density distributions. These comparisons allow to draw conclusions on the proton density distribution of the neutronrich lithium isotopes. The results show that even for the most exotic nucleus11Li the proton distribution is only very weakly influenced by the long tail in the neutron density distribution already established in several experiments. 相似文献
999.
1000.
Christian R. Noe M. Knollmüller K. Dungler C. Miculka P. Gärtner 《Monatshefte für Chemie / Chemical Monthly》1991,122(8-9):705-718
Summary Acetals3 and4 of alkylthienylcarbinols1 were prepared using an enantiomerically pure lactol2. In the key step these compounds were desulfurized. After deprotection the target compounds7 andent-7 were obtained.
Herrn Prof. Dr. H. Oelschläger zum 70. Geburtstag gewidmet 相似文献