全文获取类型
收费全文 | 20930篇 |
免费 | 940篇 |
国内免费 | 100篇 |
专业分类
化学 | 14137篇 |
晶体学 | 126篇 |
力学 | 494篇 |
数学 | 3304篇 |
物理学 | 3909篇 |
出版年
2023年 | 151篇 |
2022年 | 187篇 |
2021年 | 293篇 |
2020年 | 410篇 |
2019年 | 423篇 |
2018年 | 267篇 |
2017年 | 271篇 |
2016年 | 683篇 |
2015年 | 600篇 |
2014年 | 697篇 |
2013年 | 1140篇 |
2012年 | 1182篇 |
2011年 | 1306篇 |
2010年 | 803篇 |
2009年 | 716篇 |
2008年 | 1097篇 |
2007年 | 1093篇 |
2006年 | 1024篇 |
2005年 | 926篇 |
2004年 | 805篇 |
2003年 | 684篇 |
2002年 | 658篇 |
2001年 | 338篇 |
2000年 | 317篇 |
1999年 | 301篇 |
1998年 | 265篇 |
1997年 | 254篇 |
1996年 | 291篇 |
1995年 | 279篇 |
1994年 | 236篇 |
1993年 | 229篇 |
1992年 | 212篇 |
1991年 | 198篇 |
1990年 | 196篇 |
1989年 | 188篇 |
1988年 | 163篇 |
1987年 | 143篇 |
1986年 | 134篇 |
1985年 | 185篇 |
1984年 | 173篇 |
1983年 | 141篇 |
1982年 | 169篇 |
1981年 | 178篇 |
1980年 | 154篇 |
1979年 | 159篇 |
1978年 | 176篇 |
1977年 | 142篇 |
1976年 | 115篇 |
1975年 | 132篇 |
1974年 | 104篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
Guido Momerency Karel Van Cauwenberghe Ernst A. De Bruijn Allan T. Van Oosterom Martin S. Highley Peter G. Harper 《Journal of separation science》1994,17(9):655-661
A method is described for the determination of the antitumor agent iphosphamide and seven of its metabolites in the plasma of cancer patients by multiple ion monitoring (MIM) GC-MS, mainly using the electron capture chemical ionization mode, of stable methyl and/or trifluoroacetyl derivatives. The metabolites determined were 2- and 3-dechloroethyliphosphamide, 4-ketoiphosphamide, carboxyiphosphamide, iphosphamide mustard, and two previously undetected metabolites, chloroethylamine and 1,3-oxazolidine-2-one. The isolation of the acidic and neutral metabolites was performed by solid phase extraction on to C18 adsorbent at pH 4. The weakly acidic iphosphamide mustard, isolated under these conditions with a yield of ca 50%, was measured as a stable methyltrifluoroacetyl derivative, in contrast to the corresponding phosphoramide mustard of the isomer cyclophosphamide which decomposes during derivatization. Chloroethylamine and 1,3-oxazolidine-2-one were isolated with high yield by liquid extraction with ethyl acetate at pH 10. Selective measurement of several metabolite derivatives with similar retention times was performed by multiple ion monitoring MS of specific ion masses, using a methyl phenyl siloxane capillary column previously employed in the study of cyclophosphamide metabolites. Quantitation of metabolites in patient plasma samples could be performed in the concentration range 3 ng to 20 μg per ml of original plasma. 相似文献
73.
V. Katchkanov J.F.W. Mosselmans S. Dalmasso K.P. ODonnell S. Hernandez K. Wang R.W. Martin O. Briot N. Rousseau G. Halambalakis K. Lorenz E. Alves 《Superlattices and Microstructures》2004,36(4-6):729
The local structure of Tm3+ ions incorporated into GaN epilayers was studied by means of Extended X-ray Absorption Fine Structure. The samples were doped either in situ during growth by Molecular Beam Epitaxy or by ion implantation of layers grown by Metal Organic Chemical Vapour Deposition. The implantation was done at ion energy of 300 keV and different nominal fluences of 3×1015, 4×1015 cm−2 and 5×1015 cm−2. The concentration of Tm in the samples studied was measured by Wavelength Dispersive X-ray analysis. For the in situ doped sample with concentration of 0.5%, and for all of the implanted samples, Tm was found on the Ga site in GaN. The ion implanted sample and an in situ doped sample with a similar concentration of Tm showed the same local structure, which suggests that the lattice site occupied by Tm does not depend on the doping method. When the average Tm concentration for in situ doped samples is increased to 1.2% and 2.0%, Tm is found to occupy the Ga substitutional site and the presence of a substantial number of Tm ions in the second coordination sphere indicates dopant clustering in the films. The formation of pure TmN clusters was found in an in situ doped sample with a dopant concentration of 3.4%. 相似文献
74.
Martin Weese 《Mathematical Logic Quarterly》1992,38(1):325-326
75.
Martin Celli 《Comptes Rendus Mathematique》2005,340(7):513-518
The Note deals with rigid solutions of the N-Body Problem, i.e. solutions with constant mutual distances between the bodies. It is shown that for these motions, the configuration is balanced in the sense of Albouy and Chenciner [Invent. Math. 131 (1998) 151–184] even when the masses are of different signs. This fact was proved only for positive masses, using the scalar product they define. A consequence of the result is the constancy of the rotation velocity. It is also shown that any configuration can generate non-planar rigid motions for certain masses. Such motions do not exist with positive masses. All the results can be generalized to systems with N charged particles. To cite this article: M. Celli, C. R. Acad. Sci. Paris, Ser. I 340 (2005). 相似文献
76.
77.
Models are developed for decision making about monitoring andmaintenance of systems whose performance through time is describedby a general stochastic process. The system is monitored andpreventive and corrective maintenance actions are carried outin response to the observed system state. The decision processis simplified by using the maximum process as a decision variable.The models developed generalize age replacement models and othersimple maintenance strategies. The approach can deal with failuresthat prevent the system functioning further, and also failuresthat are defined by regulation or economic considerations. Attentionis restricted to perfect repair and inspection, but the structureprovides the framework for further developments. 相似文献
78.
Martin Breza 《Czechoslovak Journal of Physics》1992,42(2):201-207
The electronic structure of YBa2Cu3Ox (x=6 and 7) is investigated using the CNDO molecular orbital method. Electronic structures of model clusters [Cu3O10]–15, [Ba8Cu3O10]+1, [Y8Cu3O10]+9 of the non-superconducting (x=6) and [Cu3O12]–17, [Ba8Cu3O12]–1, [Y8Cu3O12]+7 of the superconducting (x=7) phases are compared. Y and Ba layers cause a considerable electron density transfer from the central Cu(1) region. 相似文献
79.
Tomomi Kawasaki Miyuki Nagaoka Tomoko Satoh Ayako Okamoto Rie Ukon Atsuyo Ogawa 《Tetrahedron》2004,60(15):3493-3503
Claisen rearrangement triggered by enolization of 2-allyloxyindolin-3-ones with DBU was performed in order to prepare 3-allyl-3-hydroxyindolin-2-ones. Total synthesis of 3-hydroxyindolin-2-one alkaloids, (±)-donaxaridine, as well as (±)-convolutamydines A and E, was achieved by transformation of the allyl moiety of 3-allyl-3-hydroxyindolin-2-ones. 相似文献
80.
Martin E. V. Pedersen Lars S. Rishøj Henrik Steffensen Sanshui Xiao Niels Asger Mortensen 《Optical and Quantum Electronics》2007,39(10-11):903-911
Slow-light enhanced optical detection in liquid-infiltrated photonic crystals is theoretically studied. Using a scattering-matrix approach and the Wigner–Smith delay time concept, we show that optical absorbance benefits both from slow-light phenomena as well as a high filling factor of the energy residing in the liquid. Utilizing strongly dispersive photonic crystal structures, we numerically demonstrate how liquid-infiltrated photonic crystals facilitate enhanced light–matter interactions, by potentially up to an order of magnitude. The proposed concept provides strong opportunities for improving existing miniaturized absorbance cells for optical detection in lab-on-a-chip systems. 相似文献