全文获取类型
收费全文 | 2219篇 |
免费 | 64篇 |
国内免费 | 15篇 |
专业分类
化学 | 1576篇 |
晶体学 | 12篇 |
力学 | 61篇 |
数学 | 363篇 |
物理学 | 286篇 |
出版年
2022年 | 22篇 |
2021年 | 35篇 |
2020年 | 25篇 |
2019年 | 26篇 |
2018年 | 25篇 |
2017年 | 17篇 |
2016年 | 33篇 |
2015年 | 57篇 |
2014年 | 42篇 |
2013年 | 113篇 |
2012年 | 127篇 |
2011年 | 140篇 |
2010年 | 81篇 |
2009年 | 63篇 |
2008年 | 114篇 |
2007年 | 115篇 |
2006年 | 132篇 |
2005年 | 90篇 |
2004年 | 94篇 |
2003年 | 81篇 |
2002年 | 67篇 |
2001年 | 45篇 |
2000年 | 21篇 |
1999年 | 13篇 |
1998年 | 20篇 |
1997年 | 17篇 |
1996年 | 36篇 |
1995年 | 19篇 |
1994年 | 18篇 |
1993年 | 29篇 |
1992年 | 15篇 |
1991年 | 23篇 |
1990年 | 20篇 |
1989年 | 19篇 |
1988年 | 23篇 |
1987年 | 13篇 |
1986年 | 15篇 |
1985年 | 29篇 |
1984年 | 25篇 |
1983年 | 15篇 |
1982年 | 28篇 |
1981年 | 27篇 |
1980年 | 16篇 |
1979年 | 34篇 |
1978年 | 20篇 |
1977年 | 30篇 |
1976年 | 22篇 |
1975年 | 24篇 |
1974年 | 25篇 |
1973年 | 18篇 |
排序方式: 共有2298条查询结果,搜索用时 15 毫秒
11.
Long-Mei Zeng Xiong Fu Jing-Yu Su Florecita De Guzman J. Francis Schmitz M. Bilayet Hossain Dick van Der Helm 《中国化学》1991,9(2):136-143
Three pyrrololactam alkaloids have been isolated from the marine sponge Phacellia fusca Schmidt collected from the South China Sea, and their structures were determined as 1–3 , on the grounds of spectral data. This is the first report of these compounds from this sponge. The structure of 1 was confirmed by X-ray analysis. 相似文献
12.
Electrostatic spring softening is an important characteristic of electrostatically actuated dual-axis micromirror, since it lowers the resonant frequencies. This paper presents an approach based on approximating the electrostatic forces by the first-order Taylor's series expansion to investigate this characteristic. The dual-axis micromirror studied in this paper has three motion modes, two torsional (about x- and y-axis, respectively) and one translational (about z-axis). The stiffnesses of all these modes are softened by a DC bias voltage applied to the mirror plate. The resonant frequencies are lowered with the increment of the bias voltage. The relationship of the bias voltage and the resonant frequencies of all the motion modes is derived. The analytical results show that the resonant frequency curves are affected by the capacitor geometries, i.e. the gap between the mirror plate and the electrodes and the electrodes size. The lowering curves drop slowly when the bias voltage is small. While for large bias voltage, the lowering curves drop rapidly. The experiment results are consistent with those obtained by the analytical approach. 相似文献
13.
14.
J. P. Leroy J. Micheli G. C. Rossi K. Yoshida 《Zeitschrift fur Physik C Particles and Fields》1990,48(4):653-661
In this paper we present a non-trivial check of the consistency of the quantization of a gauge theory with fermions (QCD) in the temporal gauge. We use the approach based on the finite time Feynman propagation kernel, in which the Gauss law is imposed as a constraint on the states by means of a functional integration over all the time independent gauge transformations acting on the boundary values of the fields. We spell out in detail the “Feynman rules” when fermions are present and we compute, as an example, the gauge invariant correlation function $$\begin{gathered} G(t) = \left\langle {\bar \psi (0,t)(\gamma _5 \gamma _0 )\frac{{1 - \gamma _0 }}{2}P} \right. \hfill \\ \left. { \cdot \exp \left( {ig\int\limits_0^t {A_0 (0,t')dt'} } \right)(\gamma _5 \gamma _0 )^ + (0,0)} \right\rangle \hfill \\ \end{gathered} $$ up to orderg 2, obtaining the expected result. 相似文献
15.
Daniel Tholen Roderick Chappel Barbara Francis 《Accreditation and quality assurance》2006,11(8-9):400-407
International requirements for PT and EQA state that providers must demonstrate that samples are homogeneous and stable. However, testing for homogeneity and stability can be expensive, use samples that could otherwise serve as quality-control materials, and can also fail to detect significant inhomogeneity and instability. In some situations it may be preferable to use the results from participants to identify problems with sample lots, if historic results follow predictable distributions and a statistical decision rule can be produced. An unusually high proportion of incorrect results may indicate that samples were inhomogeneous or unstable. Conditions under which this can be demonstrated are discussed, and the efficacy of the proposal is demonstrated with two examples. This procedure is especially effective when there are a large number of participants and/or a historic small proportion of incorrect results. Providers who adopt this proposal will need to retain samples for testing and assume the risk of distributing bad samples.Presented at the Eurachem PT Workshop September 2005, Portorož, Slovenia 相似文献
16.
Crystals of Ba5Fe5−xPtxClO13 and Ba5Co5−yPtyClO13 were prepared for x=1.31, 1.51, 1.57, 1.59 and y=0, 0.97 and 1.08 in a BaCl2 flux and investigated by X-ray diffraction methods. These compounds adopt a 10H perovskite structure built from the stacking of BaO3 and BaOCl layers in the sequence (BaO3)4(BaOCl) with space group P63/mmc. The cation sites within the trimeric unit of face-sharing octahedra are occupied by Co or Fe and Pt ions, while the tetrahedral sites formed between BaO3 and BaOCl layers are only occupied by Fe or Co. Moreover, oxygen stoichiometry indicates an average oxidation state for Co and Fe higher than +III, indicating the stabilization of Co4+ and Fe4+. 相似文献
17.
Joseph I. Degraw Lawrence F. Kelly Roy L. Kisliuk Yvette Gaumont Francis M. Sirotnak 《Journal of heterocyclic chemistry》1982,19(6):1587-1588
A synthesis of 8,10-dideazaminopterin, using 2,4-diamino-6-bromomethyl-8-deazapteridine ( 2 ) as a key intermediate, is described. Condensation of the triphenylphosphinylide derived from 2 with p-formylbenzoyl-L-glutamate afforded a 9,10-dehydro-8,10-dideazaminopterin ester intermediate 5 . Hydrogenation of the olefinic linkage and subsequent hydrolysis of the glutamate ester gave the title compound. 8,10-Dideazaminopterin was a potent growth inhibitor of folate dependent bacteria. It was 16 times more potent then methotrexate as an inhibitor of dihydrofolate reductase derived from L1210 leukemia cells, and showed strong activity against L1210 in mice. 相似文献
18.
19F and (119)Sn MAS NMR spectroscopy have been used to investigate the fluoride ion conductor, BaSnF(4), a member of the MSnF(4) family of fluorite-related anionic conductors containing double layers of Sn(2+) and M(2+) cations. Two fluorine sublattices were observed by (19)F MAS NMR, which could be assigned to specific sites in the lattice. The first sublattice is due to fluorine atoms located in Ba(2+) double layers and is rigid on the MAS NMR time scale at room temperature. The second sublattice comprises the fluoride ions between the Ba(2+) and Sn(2+) layers, and the few fluorine atoms that inhabit the Sn(2+)-Sn(2+) double layers. These ions are in rapid exchange with each other, and an extremely short correlation time tau(C) for the motion of these ions of <3 x 10(-)(5) s is obtained at -100 degrees C. T(1) measurements indicate that tau(C) approaches 10(-)(8) s at room temperature. (19)F-to-(119)Sn cross-polarization (CP) experiments confirmed the assignments of the resonances, and that the fluorine atoms located next to the tin atoms are extremely mobile at room temperature (and thus do not contribute to the CP process). Two-dimensional (19)F exchange experiments showed that exchange between the rigid and mobile lattice does occur, but at a much slower rate (tau(C) approximately 10 ms at 250 degrees C). Low-temperature (19)F MAS and (19)F-to-(119)Sn CP NMR spectra demonstrate that the motion of the fluoride ions has almost completely frozen out by -150 degrees C. The results are consistent with rapid two-dimensional (anisotropic) conductivity involving the fluoride ions between the Ba and Sn layers. Conductivity in three dimensions requires hops between the ions in the BaF(2)-like layers and the mobile ions. This process does occur, but with exchange rates that are at least 6-7 orders of magnitude slower. 相似文献
19.
Francis Garnier Pierre Krausz Jacques-Emile Dubois 《Journal of organometallic chemistry》1979,170(2):195-201
The photoinduced metathesis [W(CO)6CCl4hν] of cis- and trans-RCH=CHMe gives 2-butene in a very high trans but low cis stereoselectivity. Propene behaves like a cis olefin. These results are consistent with the proposal of a dinuclear tungsten intermediate, which requires high steric constraints in the transalkylidenation step. 相似文献
20.
Francis D. de Kőrösy 《Mikrochimica acta》1989,97(1-2):25-29
Donor or acceptor compounds in apolar organic solvents become charged at a high-field strength electrode and are driven to a surrounding low-field strength electrode of opposite sign. At 10 kV per cm the solutions conduct 3–7 times better when the highly charged electrode is positive in donor solutions. The opposite situation occurs in acceptor solutions. 相似文献