全文获取类型
收费全文 | 69953篇 |
免费 | 5772篇 |
国内免费 | 11459篇 |
专业分类
化学 | 45975篇 |
晶体学 | 1878篇 |
力学 | 686篇 |
综合类 | 422篇 |
数学 | 8412篇 |
物理学 | 13481篇 |
综合类 | 16330篇 |
出版年
2024年 | 187篇 |
2023年 | 1026篇 |
2022年 | 1932篇 |
2021年 | 1945篇 |
2020年 | 2019篇 |
2019年 | 1985篇 |
2018年 | 1732篇 |
2017年 | 2060篇 |
2016年 | 2169篇 |
2015年 | 2043篇 |
2014年 | 2897篇 |
2013年 | 5331篇 |
2012年 | 4208篇 |
2011年 | 4668篇 |
2010年 | 4085篇 |
2009年 | 4936篇 |
2008年 | 4732篇 |
2007年 | 5092篇 |
2006年 | 4632篇 |
2005年 | 3933篇 |
2004年 | 3695篇 |
2003年 | 3154篇 |
2002年 | 2596篇 |
2001年 | 2162篇 |
2000年 | 2023篇 |
1999年 | 1766篇 |
1998年 | 1393篇 |
1997年 | 1228篇 |
1996年 | 1089篇 |
1995年 | 1071篇 |
1994年 | 978篇 |
1993年 | 816篇 |
1992年 | 737篇 |
1991年 | 570篇 |
1990年 | 394篇 |
1989年 | 354篇 |
1988年 | 292篇 |
1987年 | 200篇 |
1986年 | 137篇 |
1985年 | 136篇 |
1984年 | 120篇 |
1983年 | 55篇 |
1982年 | 87篇 |
1981年 | 108篇 |
1980年 | 74篇 |
1979年 | 84篇 |
1978年 | 62篇 |
1977年 | 62篇 |
1976年 | 36篇 |
1973年 | 34篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
Various 2-substituted indoles were prepared by heteroannulation of o-iodoanilines and terminal alkynes in a one-pot reaction with a Pd(II)-NaY zeolite catalyst. The product formation largely depended on the solvent, base, and reaction temperature. The recycled catalyst showed good reusability in the heteroannulation reaction. 相似文献
82.
T.S. Khlebnicova V.G. Isakova A.V. Baranovsky E.V. Borisov F.A. Lakhvich 《Journal of fluorine chemistry》2006,127(12):1564-1569
A one-pot synthesis of 2-perfluoroalkanoylcyclohexane-1,3-diones via C-acylation of cyclohexane-1,3-diones with N-perfluoroacylimidazole as an acylating agent is reported. A reaction was examined with isolated N-trifluoroacetylimidazole and with N-perfluoroacylimidazoles generated in situ from perfluorocarboxylic acid anhydrides or perfluorocarboxylic acids. 相似文献
83.
A graph G is N2‐locally connected if for every vertex ν in G, the edges not incident with ν but having at least one end adjacent to ν in G induce a connected graph. In 1990, Ryjá?ek conjectured that every 3‐connected N2‐locally connected claw‐free graph is Hamiltonian. This conjecture is proved in this note. © 2004 Wiley Periodicals, Inc. J Graph Theory 48: 142–146, 2005 相似文献
84.
Y.B. Melnichenko G.D. Wignall D.R. Cole H. Frielinghaus L.A. Bulavin 《Journal of Molecular Liquids》2005,120(1-3):7-9
Small angle neutron scattering (SANS) is a well-established technique for investigating the behavior of confined binary liquid solutions, as it can probe the correlation length and susceptibility in pores on length scales 1 – 100 nm. We applied SANS to explore the influence of confinement on critical behavior of an individual fluid carbon dioxide (CO2) in a highly porous aerogel. The results demonstrate that quenched disorder induced by aerogel significantly depresses density fluctuations. Despite the negligible volume occupied by aerogel (< 4%), the macroscopic phase separation of confined CO2 into coexisting liquid and gaseous phases is suppressed and below the critical temperature of the bulk fluid frozen methastable microdomains are formed. Experimental data show that critical adsorption is as important as the effect of confinement in defining the behavior of confined fluids. 相似文献
85.
86.
This paper is devoted to a detailed theoretical study of an ion pair SN2 reaction LiNCO+CH3F in the gas phase and in solution at the level of MP2(full)/6-31+G**//HF/6-31+G**. Two possible reaction mechanisms, inversion and retention, are discussed. There are eight possible reaction pathways. The inversion mechanism is more favorable no matter in the gas phase or in solution based on analyses of the transition structures. Methyl isocyanate should form preferentially in the gas phase and more stable methyl cyanate is the main product in solution. The retardation of the reaction in solvents was attributed to the difference in solvation in the separated reactants and in the transition state. 相似文献
87.
A Promising MoO_x-based Catalyst for n-Heptane Isomerization 总被引:1,自引:0,他引:1
Song Hai CHAI Xin Ping WANG* Ying Jun WANG Tian Xi CAI State Key Laboratory of Fine Chemicals Dalian University of Technology Dalian 《中国化学快报》2003,14(7)
The increasing demand for higher-octane gasoline and the regulations limiting the amount of aromatics in the fuel motivate the interest in catalytic isomerization of n-alkanes. In the last ten years, transition metal oxides or oxycarbides based on molybdenum or tungstate have attracted much attention due to their high activity and isomerization selectivity compared to the conventional bifunctional supported platinum catalyst and high resistance to sulphur and nitrogen catalyst poisons1-5. Ma… 相似文献
88.
89.
90.
Franz-Viktor Kuhlmann 《Advances in Mathematics》2004,188(2):399-424
We consider the Zariski space of all places of an algebraic function field F|K of arbitrary characteristic and investigate its structure by means of its patch topology. We show that certain sets of places with nice properties (e.g., prime divisors, places of maximal rank, zero-dimensional discrete places) lie dense in this topology. Further, we give several equivalent characterizations of fields that are large, in the sense of F. Pop's Annals paper Embedding problems over large fields. We also study the question whether a field K is existentially closed in an extension field L if L admits a K-rational place. In the appendix, we prove the fact that the Zariski space with the Zariski topology is quasi-compact and that it is a spectral space. 相似文献