全文获取类型
收费全文 | 36934篇 |
免费 | 3966篇 |
国内免费 | 6317篇 |
专业分类
化学 | 30121篇 |
晶体学 | 1403篇 |
力学 | 828篇 |
综合类 | 256篇 |
数学 | 3871篇 |
物理学 | 10738篇 |
出版年
2024年 | 56篇 |
2023年 | 367篇 |
2022年 | 848篇 |
2021年 | 946篇 |
2020年 | 1163篇 |
2019年 | 1152篇 |
2018年 | 967篇 |
2017年 | 1312篇 |
2016年 | 1334篇 |
2015年 | 1210篇 |
2014年 | 1623篇 |
2013年 | 3061篇 |
2012年 | 2054篇 |
2011年 | 2615篇 |
2010年 | 2170篇 |
2009年 | 2643篇 |
2008年 | 2709篇 |
2007年 | 2609篇 |
2006年 | 2501篇 |
2005年 | 2235篇 |
2004年 | 1939篇 |
2003年 | 1599篇 |
2002年 | 1495篇 |
2001年 | 1235篇 |
2000年 | 1152篇 |
1999年 | 902篇 |
1998年 | 786篇 |
1997年 | 654篇 |
1996年 | 593篇 |
1995年 | 583篇 |
1994年 | 530篇 |
1993年 | 428篇 |
1992年 | 337篇 |
1991年 | 240篇 |
1990年 | 154篇 |
1989年 | 148篇 |
1988年 | 129篇 |
1987年 | 86篇 |
1986年 | 90篇 |
1985年 | 75篇 |
1984年 | 65篇 |
1983年 | 38篇 |
1982年 | 52篇 |
1981年 | 54篇 |
1980年 | 60篇 |
1979年 | 56篇 |
1978年 | 45篇 |
1977年 | 39篇 |
1976年 | 20篇 |
1973年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
71.
Three N-3-phenyl-2-propenoyl amino acids were synthesized through interaction between 3-phenyl-2-propenoyl chloride and amino acids in alkaline with a high Yield.Structure of the products were identified by elemental analysis,IR and NMR spectroscopy. 相似文献
72.
The catalytic activities of SO42-/TiO2-MoO3 in synthesizing cyclohexanone ethylene ketal,cyclohexanone 1,2-propanediol ketal, 2-propyl-1,3-dioxolane, 4-methyl-2-isopropyl-1,3-dioxolane,2-isopropyl-1,3-dioxolane, 4-methyl-2-isopropyl-1,3-dioxolane, butanone ethy-lene ketal and butanone 1,2-propanediol ketal were reported. It has been demonstrated that SO42-/TiO2-MoO3 is an excellent catalyst. Various factors concerned in this reaction have been investigated. The optimum conditions have been found, that is, the molar ratio of aldehyde/ketone to alcohol was 1:1.5 or 1:1.3,the mass ratio of the catalyst used to the reactants was 0.25~1.5%, and the reaction time was 45~60 min. Under this conditions, the yield of cyclohexanone ethylene ketal is 82.7%, cyclohexanone 1,2-propanediol ketal is 83.4%, the yield of 2-propyl-1,3-dioxolane is 68.1%,4-methyl-2-isopropyl-1,3-dioxolane is 87.5%, the yield of 2-isopropyl-1,3-dioxolane is 70.7%,4-methyl-2-isopropyl-1,3-dioxolane is 82.5%, the yield of butanone ethylene ketal is 74.1%, and butanone 1,2-propanediol ketal is 94.9%.Some equation and experiment results concerned of the synthetic acetals or ketals were listed as follows. 相似文献
73.
2H-1,2,3-Diazaphospholes 3a-3h were prepared from various ketone acylhydrazones and phosphorus trichloride in the presence of triethylamine. Compounds 3 underwent feasible hetero-Diels-Alder reactions with cyclopentadiene to afford the respective anellated [1,2,3]diazaphospholes 4a--4d as well as 4a‘--4d‘ in moderate yields. The endo rule in the reaction was observed under kinetic control conditions. 相似文献
74.
WeiMingXU LuLingWU XianHUANG 《中国化学快报》2004,15(11):1279-1280
A mild, regioselective 1, 3-dipolar cycloaddition protocol for the preparation ofphenylselenomethyl isoxazolines through substituted allyl phenyl selenides and nitrile oxides wasreported. 相似文献
75.
Solid Phase Synthesis of 2-Substituted 1,3-Oxazin-6-ones Using Resin-bound Cyclic Malonic Acid Ester
A facile solid phase synthesis of 2-substituted 1,3-oxazin-6-ones using polymer-supported Meldrum‘s acid has been reported. Reaction of the resin-bound cyclic malonic acid ester with triethyl orthoformate and subsequent double substitution with amide, afforded the corresponding polymer-supported acylaminomethylene cyclic malonic acid ester, which upon thermal treatment led to 1, 3-oxazin-6-ones in good yields and with high purity. 相似文献
76.
首先叙述了光纤传输Nd~(3+): YAG激光的优点,接着分析了不同光纤和其传输Nd~(3+): YAG激光的特点。然后通过对光纤传输系统各部分的分析与设计,结合实际,设计了光纤耦合机构。最后,对来自于工件反射光问题提出了解决方案。 相似文献
77.
Mean value in invexity analysis 总被引:1,自引:0,他引:1
In this paper, a generalization of the mean value theorem is considered in the case of functions defined on an invex set with respect to η (which is not necessarily connected). 相似文献
78.
79.
该文讨论了形变诺振子相干态的一些有意义的性质,利用量子代数SUq(3)的形变谐振子实现,构造了SU3(q)在玻色、费米两种情况下的相干态. 相似文献
80.