全文获取类型
收费全文 | 326069篇 |
免费 | 3877篇 |
国内免费 | 1643篇 |
专业分类
化学 | 157284篇 |
晶体学 | 5209篇 |
力学 | 17380篇 |
综合类 | 89篇 |
数学 | 54711篇 |
物理学 | 96916篇 |
出版年
2021年 | 2031篇 |
2020年 | 2265篇 |
2019年 | 2449篇 |
2018年 | 12938篇 |
2017年 | 12767篇 |
2016年 | 10341篇 |
2015年 | 3746篇 |
2014年 | 4891篇 |
2013年 | 11657篇 |
2012年 | 12359篇 |
2011年 | 20927篇 |
2010年 | 13028篇 |
2009年 | 13455篇 |
2008年 | 15885篇 |
2007年 | 17645篇 |
2006年 | 8950篇 |
2005年 | 9064篇 |
2004年 | 8536篇 |
2003年 | 8351篇 |
2002年 | 7170篇 |
2001年 | 8601篇 |
2000年 | 6567篇 |
1999年 | 4872篇 |
1998年 | 3870篇 |
1997年 | 3778篇 |
1996年 | 3553篇 |
1995年 | 3251篇 |
1994年 | 3004篇 |
1993年 | 2790篇 |
1992年 | 3575篇 |
1991年 | 3512篇 |
1990年 | 3349篇 |
1989年 | 3265篇 |
1988年 | 3271篇 |
1987年 | 3295篇 |
1986年 | 3097篇 |
1985年 | 3989篇 |
1984年 | 3976篇 |
1983年 | 3061篇 |
1982年 | 3191篇 |
1981年 | 3207篇 |
1980年 | 2946篇 |
1979年 | 3375篇 |
1978年 | 3387篇 |
1977年 | 3499篇 |
1976年 | 3286篇 |
1975年 | 2970篇 |
1974年 | 2939篇 |
1973年 | 2857篇 |
1972年 | 1919篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
X Y Liu C J Li 《Science in China. Series B, Chemistry, life sciences & earth sciences》1989,32(10):1215-1223
To further evaluate the causality between endogenous pyrogen (EP)-induced fever and cyclic adenosine-3',5'-monophosphate (cyclic AMP) level, the effects of sodium salicylate (SS) on the febrile response and increased levels of cyclic AMP in both cerebrospinal fluid (c.s.f.) and plasma during EP-induced fever in rabbits were observed. The results suggest that cyclic AMP is probably involved in the central mediation of EP-induced fever and that increased concentration of cyclic AMP in c.s.f. associated with EP-induced fever is not the result of temperature elevation but appears to be caused by the increased synthesis in the central nervous system. In addition it is confirmed that blood is impossibly a contributory source of increased cyclic AMP in c.s.f. during EP fever, and that SS may act subsequent to the increase in cyclic AMP. 相似文献
12.
K. C. Kumara Swamy E. Balaraman M. Phani Pavan N. N. Bhuvan Kumar K. Praveen Kumar N. Satish Kumar 《Journal of Chemical Sciences》2006,118(6):495-501
The diversity of products in the reaction of diethyl azodicarboxylate (DEAD)/diisopropyl azodicarboxylate (DIAD) and activated
acetylenes with PIII compounds bearing oxygen or nitrogen substituents is discussed. New findings that are useful in understanding the nature
of intermediates involved in the Mitsunobu reaction are highlighted. X-ray structures of two new compounds (2-t-Bu-4-MeC6H3O)P (μ-N-t-Bu)2P+[(NH-t-Bu)N[(CO2]-i-Pr)(HNCO2-i-Pr)]](Cl-)(2-t-Bu-4-MeC6H3OH)(23)and [CH2(6-t-Bu-4-Me-C6H2O)2P(O)C(CO2Me)C-(CO2Me)CClNC(O)Cl] (33) are also reported. The structure of23 is close to one of the intermediates proposed in the Mitsunobu reaction. 相似文献
13.
14.
Francis K. Bell 《Journal of Graph Theory》2003,43(2):137-149
It is shown that, if t is an integer ≥3 and not equal to 7 or 8, then there is a unique maximal graph having the path Pt as a star complement for the eigenvalue ?2. The maximal graph is the line graph of Km,m if t = 2m?1, and of Km,m+1 if t = 2m. This result yields a characterization of L(G ) when G is a (t + 1)‐vertex bipartite graph with a Hamiltonian path. The graphs with star complement Pr ∪ Ps or Pr ∪ Cs for ?2 are also determined. © 2003 Wiley Periodicals, Inc. J Graph Theory 43: 137–149, 2003 相似文献
15.
K. Murawski 《Waves in Random and Complex Media》2004,14(3):467-477
The effect of space- and time-dependent random mass density, velocity, and pressure fields on frequencies and amplitudes of acoustic waves is considered by means of the analytical perturbative method. The analytical results, which are valid for weak fluctuations and long wavelength sound waves, reveal frequency and amplitude alteration, the effect of which depends on the type of random field. In particular, the effect of a random mass density field is to increase wave frequencies. Space-dependent random velocity and pressure fields reduce wave frequencies. While space-dependent random fields attenuate wave amplitudes, their time-dependent counterparts lead to wave amplification. In another example, sound waves that are trapped in the vertical direction but are free to propagate horizontally are affected by a space-dependent random mass density field. This effect depends on the direction along which the field is varying. A random field, which varies along the horizontal direction, does not couple vertically standing modes but increases their frequencies and attenuates amplitudes. These modes are coupled by a random field which depends on the vertical coordinate, but the dispersion relation remains the same as in the case of the deterministic medium. 相似文献
16.
17.
A one-dimensional bulk reaction model for the oxidation of nickeltitanium is formulated, with preferential oxidation of titaniumbeing included. The modelling is directed at the better understandingof the dominant mechanisms involved in the oxidation processand their significance for the biocompatibility of the alloy.Two different regimes for the relative diffusivities of oxygenand the metals are investigated. By assuming fast bulk reactions,different asymptotic structures emerge in different parameterregimes and the resulting models take the form of moving boundaryproblems. Different profiles of nickel concentration are obtained:in particular a nickel-rich layer (observed in practice) ispresent below the oxide/metal interface for the case when oxygenand the metals diffuse at comparable rates. 相似文献
18.
D. Amutha Rani Y. Yamamoto S. Mohri M. Sivakumar Y. Tsujita H. Yoshimizu 《Journal of Polymer Science.Polymer Physics》2003,41(3):269-273
In this study, a novel stepwise extraction method has been examined. The guest molecules housed between the helices of the clathrate δ form of syndiotactic polystyrene can be removed completely with this method. A systematic study of the preparation of a solvent‐free mesophase (emptied clathrate) membrane, its helical and residual solvent contents, and its structural transformations has been performed. In this first attempt, an enhancement in the TTGG helical content has been observed in the extracted membrane, and a conceptual mechanism is proposed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 269–273, 2003 相似文献
19.
In previous papers by the present author, a machinery for calculating automorphisms, constructing invariants, and classifying real submanifolds of a complex manifold was developed. The main step in this machinery is the construction of a “nice” model surface. The nice model surface can be treated as an analog of the osculating paraboloid in classical differential geometry. Model surfaces suggested earlier possess a complete list of the desired properties only if some upper estimate for the codimension of the submanifold is satisfied. If this estimate fails, then the surfaces lose the universality property (that is, the ability to touch any germ in an appropriate way), which restricts their applicability. In the present paper, we get rid of this restriction: for an arbitrary type (n,K) (where n is the dimension of the complex tangent plane, and K is the real codimension), we construct a nice model surface. In particular, we solve the problem of constructing a nondegenerate germ of a real analytic submanifold of a complex manifold of arbitrary given type (n,K) with the richest possible group of holomorphic automorphisms in the given class. 相似文献
20.
The title compound is a centrosymmetric dimer with each cadmium in a distorted CdS5 square pyramidal geometry. The Cd–S bond distances range from 2.5626(11) to 2.8459(11) Å. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献