全文获取类型
收费全文 | 225371篇 |
免费 | 2045篇 |
国内免费 | 712篇 |
专业分类
化学 | 124929篇 |
晶体学 | 3374篇 |
力学 | 8575篇 |
综合类 | 6篇 |
数学 | 24794篇 |
物理学 | 66450篇 |
出版年
2020年 | 2112篇 |
2019年 | 2394篇 |
2018年 | 3308篇 |
2017年 | 3218篇 |
2016年 | 4462篇 |
2015年 | 2565篇 |
2014年 | 4077篇 |
2013年 | 9714篇 |
2012年 | 7478篇 |
2011年 | 9167篇 |
2010年 | 6449篇 |
2009年 | 6241篇 |
2008年 | 8802篇 |
2007年 | 8874篇 |
2006年 | 8318篇 |
2005年 | 7651篇 |
2004年 | 6786篇 |
2003年 | 6136篇 |
2002年 | 5978篇 |
2001年 | 6073篇 |
2000年 | 4815篇 |
1999年 | 3589篇 |
1998年 | 3146篇 |
1997年 | 3177篇 |
1996年 | 2981篇 |
1995年 | 2411篇 |
1994年 | 2520篇 |
1993年 | 2552篇 |
1992年 | 2684篇 |
1991年 | 2733篇 |
1990年 | 2609篇 |
1989年 | 2582篇 |
1988年 | 2521篇 |
1987年 | 2484篇 |
1986年 | 2403篇 |
1985年 | 3122篇 |
1984年 | 3270篇 |
1983年 | 2615篇 |
1982年 | 2888篇 |
1981年 | 2784篇 |
1980年 | 2573篇 |
1979年 | 2771篇 |
1978年 | 2881篇 |
1977年 | 2893篇 |
1976年 | 2865篇 |
1975年 | 2722篇 |
1974年 | 2696篇 |
1973年 | 2837篇 |
1972年 | 1892篇 |
1967年 | 1833篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Yu. M. Kargin E. I. Gritsenko V. V. Yanilkin V. V. Plemenkov L. K. Dubovik N. I. Maksimyuk B. M. Garifullin Sh. K. Letypov A. V. Il'yasov 《Russian Chemical Bulletin》1992,41(9):1572-1579
The electrochemical reduction of 1,1-dihalo-2-R-2-methylcyclopropanes was studied by polarography and preparative electrolysis. A mixture of stereoisomeric monoboro- and monochlorocyclopropanes was obtained with preparative yield of 60–70% in preparative electroreduction in methanol against a background of 0.1 M LiClO4. In the case of bromine derivatives of cyclopropanes (except when R = CN) an effect was found on the part of the current density on the ratio of cis and trans isomers, which was interpreted as a change, in dependence on current density, of the contributions of the reactions of reduction of the starting compounds (SN2 mechanism) and ionic pairs (SN1 mechanism). The effect of the solvent (CHCl3, DMF, DMSO, MeOH) and background salt (LiClO4, Et4NBr) on the ratio of stereoisomers is in agreement with this interpretation.A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Science Center, Russian Academy of Sciences, Kazan. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 9, pp. 2023–2032, September, 1992. 相似文献
992.
993.
H. Postma J. D. Bowman C. D. Bowman J. E. Bush P. P. J. Delheij C. M. Frankle C. R. Gould D. G. Haase J. Knudson G. E. Mitchell S. Penttilä N. R. Roberson S. J. Seestrom J. J. Szymansky S. H. Yoo V. W. Yuan X. Zhu 《Hyperfine Interactions》1992,75(1-4):153-163
Parity violation effects have been studied at 40 neutron p-wave resonances of the even-even nuclei238U and232Th. Of these 11 show parity violation effects larger than 2 standard deviations, making parity violation a rather common phenomenon.
Parity mixing up to 10% has been found. The root-mean squared matrix elements for parity violation derived from these resonances
are M=0.58 (+0.50/-0.25) meV for238U, respectively 1.39 (+0.35/-0.38) meV for232Th. 相似文献
994.
I. I. Antakov M. A. Moiseev E. V. Sokolov E. V. Zasypkin 《International Journal of Infrared and Millimeter Waves》1994,15(5):873-887
Using self-consistent field theory, the interaction of electron flow with HF fields in a gyroklystron has been analized. Optimal parameters of its output cavity corresponding to the maximum efficiency have been found. Experimental study of dependence of the efficiency on Q-factor and length of the output cavity has been carried out. The results of simulation are in good qualitative agreement with experimental data. 相似文献
995.
M M Zuk B D Rihter M E Kenney M A Rodgers M Kreimer-Birnbaum 《Journal of chromatography. A》1991,568(2):437-444
Bis(di-isobutyl octadecylsiloxy)silicon 2,3-naphthalocyanine (isoBOSINC) is a synthetic potential photosensitizer for tumor therapy. A new method, which combines solvent extraction and several purification steps, has been developed to determine its presence in tissues. Separation and quantitation of isoBOSINC is done by high-performance liquid chromatography on a silica column with toluene as a mobile phase and using fluorescence detection (lambda ex = 365 nm, lambda em = 750 nm). For recovery studies, isoBOSINC was added to muscles at levels of 0.067 and 0.67 micrograms/g; the mean recoveries were 100%, with coefficients of variation of 6.1 and 6.4%, respectively. For liver samples, the amounts added were 0.67 and 6.7 micrograms/g and for serum 0.67 and 6.70 micrograms/ml. The mean recoveries for liver were 86 and 93%, with coefficients of variation of 7.7 and 4.4%, respectively. For serum, the mean recoveries were 99 and 96%, with coefficients of variation of 2.6 and 6.9%, respectively. Due to its low detection limit and selectivity, the method is appropriate for pharmacokinetic as well as tumor uptake studies following in vivo exposure to isoBOSINC. Preliminary data on tissue distribution of the photosensitizer in normal rats are also presented. 相似文献
996.
The study of Wiener-Levinson digital filters leads to certain classes of polynomials orthogonal on the unit circle (Szeg
polynomials). Here we present theorems that show that the unknown frequencies in a periodic discrete time signal can be determined from the limiting behavior (as N → ∞) of the zeros of fixed degree Szeg
polynomials that are orthogonal with respect to a distribution defined from N successive samples of the signal. This proves an essential part of a conjecture due to Jones, Njåstad, and Saff concerning the frequency analysis problem. 相似文献
997.
E. Celeghini R. Giachetti A. Reyman E. Sorace M. Tarlini 《Letters in Mathematical Physics》1991,23(1):45-49
Quantum pseudo-orthogonal groups SO
q
(n+1,n–1) are defined as real forms of quantum orthogonal groups SO
q
(n+1,n–1) by means of a suitable antilinear involution. In particular, the casen=2 gives a quantized Lorentz group. 相似文献
998.
New concepts for the study of incompressible plane or axisymmetric flows are analysed by the stream tube method. Flows without eddies and pure vortex flows are considered in a transformed domain where the mapped streamlines are rectilinear or circular. The transformation between the physical domain and the computational domain is an unknown of the problem. In order to solve the non-linear set of relevant equations, we present a new algorithm based on a trust region technique which is effective for non-convex optimization problems. Experimental results show that the new algorithm is more robust compared to the Newton-Raphson method. 相似文献
999.
1000.
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