全文获取类型
收费全文 | 51972篇 |
免费 | 11419篇 |
国内免费 | 2826篇 |
专业分类
化学 | 51678篇 |
晶体学 | 475篇 |
力学 | 1394篇 |
综合类 | 85篇 |
数学 | 4219篇 |
物理学 | 8366篇 |
出版年
2023年 | 333篇 |
2022年 | 460篇 |
2021年 | 724篇 |
2020年 | 1794篇 |
2019年 | 3057篇 |
2018年 | 1456篇 |
2017年 | 1078篇 |
2016年 | 3938篇 |
2015年 | 4103篇 |
2014年 | 4133篇 |
2013年 | 5063篇 |
2012年 | 4282篇 |
2011年 | 3630篇 |
2010年 | 3767篇 |
2009年 | 3690篇 |
2008年 | 3405篇 |
2007年 | 2723篇 |
2006年 | 2328篇 |
2005年 | 2326篇 |
2004年 | 2044篇 |
2003年 | 1835篇 |
2002年 | 2428篇 |
2001年 | 1769篇 |
2000年 | 1624篇 |
1999年 | 688篇 |
1998年 | 317篇 |
1997年 | 267篇 |
1996年 | 312篇 |
1995年 | 246篇 |
1994年 | 228篇 |
1993年 | 198篇 |
1992年 | 187篇 |
1991年 | 162篇 |
1990年 | 191篇 |
1989年 | 148篇 |
1988年 | 129篇 |
1987年 | 93篇 |
1986年 | 96篇 |
1985年 | 118篇 |
1984年 | 86篇 |
1983年 | 84篇 |
1982年 | 65篇 |
1981年 | 58篇 |
1980年 | 55篇 |
1979年 | 55篇 |
1978年 | 47篇 |
1977年 | 52篇 |
1975年 | 48篇 |
1974年 | 55篇 |
1973年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
201.
研究了应用于锂二次电池正极的新型高能量密度存贮材料Li(AlxCo1-x)O2 (x=01—05)的磁性.发现Al3+的掺杂可导致Co3+中d电子自旋态发生变化,即有部分d电子进入高自旋态.伴随Co3+中电子状态的改变,材料结构演化也发生了相应变化,表现为c/a比增大明显减缓,较好地解释了材料结构对Vegard定律的正偏离.这对材料的微观结构与性能设计具有重要意义.
关键词:
锂电池材料
Li(AlxCo1-x)O2
磁性
自旋态
结构演化 相似文献
202.
Klai Nadia Berredjem Malika Khettache Nawel Belghit Med Yazid Rgaïnia Zine Nour‐Eddine Aouf 《Journal of heterocyclic chemistry》2004,41(1):57-60
Dedicated to the memory of Pr. Ladjama Daif A series of chiral cyclosulfamides have been synthesized by alkaline cyclisation starting from N‐benzoylamino acids (Ala, Val, Leu, Phe) derivatives and chlorosulfonyl isocyanate. A simplified and regioselective deprotection of the cyclic compounds (cyclosulfamides) containing the tert‐butyloxycarbonyl group (Boc) has been achieved in good yield by fusion under reduced pressure. 相似文献
203.
Andrew Bach Xinglong Jiang Joseph Mckenna Kapa Prasad Oljan Repic Wen‐Chung Shieh 《Journal of heterocyclic chemistry》2004,41(4):637-640
An efficient approach has been developed for the synthesis of an isofervenulin analogue 1 employing a one‐pot condensation‐substitution reaction of a chlorocarboethoxytriazine (electrophile) with a urea (nucleophile). The resulting cyclization reaction resulted in the synthesis of a pyrimido‐heterocycle in good yield in either acidic or basic media. The former was assisted by utilizing trimethylsilyl chloride. 相似文献
204.
Crystal Structure of the Zinc Amide Zn[N(SiMe3)2]2 X‐ray quality crystals of Zn[N(SiMe3)2]2 (monoclinic, P21/c) are obtained by sublimation of the zinc amide Zn[N(SiMe3)2]2 at —30 °C in vacuo (300 torr). According to the result of the X‐ray structural analysis, Zn[N(SiMe3)2]2 contains an almost linear N‐Zn‐N unit with two short N‐Zn bonds. 相似文献
205.
5‐Methylene‐2‐oxazolidinone (1) and 5‐methylene‐1,3‐thiazolidine‐2‐thione (4) react with various isocyanates to give the corresponding urethanes 3 and 5 in high yields in the presence of palladium(0) or palladium(II) catalyst under mild reaction conditions. A mechanism is proposed. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
206.
It is well known that topology and dynamics are two major aspects to determine the function of a network. We study one of the dynamic properties of a network: trajectory convergence, i.e. how a system converges to its steady state. Using numerical and analytical methods, we show that in a logical-like dynamical model, the occurrence of convergent trajectory in a network depends mainly on the type of the fixed point and the ratio between activation and inhibition links. We analytically proof that this property is induced by the competition between two types of state transition structures in phase space: tree-like transition structure and star-like transition structure. We show that the biological networks, such as the cell cycle network in budding yeast, prefers the tree-like transition structures and suggest that this type of convergence trajectories may be universal. 相似文献
207.
Chirped fiber grating was used in dispersion compensator. We tried to use the same phase mask to write fiber grating of different wavelength with two methods to reduce the price of the fiber grating. 相似文献
208.
209.
The structure of the title adduct comprises a phenanthroline derivative 2-phenyl-imidazo[4,5-f]1,10-phenanthroline and a methanol.The composition of the crystalline adduct was characterized as C19H12N4.CH3OH.It belongs to orthorhombic system,space group Pna21 with a=1.3693(4)nm,b=2.2988(7)nm,c=0.51338(15)nm,V=1.6160(8)nm^3.Z=4,and final R1=0.0423.wR2=0.1012 .Crystal structure shows that all the 19 carbon atoms and 4 nitrogen atoms are coplanar.The bond length data indicated that a very extensive conjugation system was formed.This conjugation makes the compound being a potentially excellent energy transformer used for luminescent materials. 相似文献
210.
Hong‐Qing Wang Zhao‐Jie Liu Li‐Min Yang Ming‐Wu Ding 《Journal of heterocyclic chemistry》2004,41(3):393-397
A series of new 1H‐pyrazolo[3,4‐d]pyrimidin‐4(5H)‐one Derivatives 5 has been designed and regio‐selectively synthesized via a tandem aza‐Wittig reaction. The structures of all compounds prepared have been confirmed by 1H NMR, IR, EI‐MS spectroscopy and elemental analyses. The results of preliminary bioassay indicate that most compounds 5 possess an inhibition effect against Botrytis cinereapers and Pyricularia oryzae at the concentration of 50 mg/L. 相似文献