全文获取类型
收费全文 | 3423篇 |
免费 | 150篇 |
国内免费 | 1004篇 |
专业分类
化学 | 4311篇 |
晶体学 | 26篇 |
力学 | 7篇 |
综合类 | 27篇 |
数学 | 3篇 |
物理学 | 203篇 |
出版年
2024年 | 3篇 |
2023年 | 16篇 |
2022年 | 40篇 |
2021年 | 59篇 |
2020年 | 48篇 |
2019年 | 84篇 |
2018年 | 55篇 |
2017年 | 79篇 |
2016年 | 97篇 |
2015年 | 86篇 |
2014年 | 130篇 |
2013年 | 287篇 |
2012年 | 166篇 |
2011年 | 210篇 |
2010年 | 171篇 |
2009年 | 198篇 |
2008年 | 243篇 |
2007年 | 215篇 |
2006年 | 213篇 |
2005年 | 219篇 |
2004年 | 224篇 |
2003年 | 164篇 |
2002年 | 179篇 |
2001年 | 142篇 |
2000年 | 117篇 |
1999年 | 114篇 |
1998年 | 153篇 |
1997年 | 111篇 |
1996年 | 106篇 |
1995年 | 117篇 |
1994年 | 113篇 |
1993年 | 92篇 |
1992年 | 64篇 |
1991年 | 72篇 |
1990年 | 39篇 |
1989年 | 54篇 |
1988年 | 20篇 |
1987年 | 23篇 |
1986年 | 18篇 |
1985年 | 9篇 |
1984年 | 15篇 |
1983年 | 4篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
排序方式: 共有4577条查询结果,搜索用时 31 毫秒
991.
Christopher J. Arnusch 《Tetrahedron letters》2004,45(21):4153-4156
The synthesis and NMR analysis of a novel highly constrained scaffold is described. The 14-membered macrocyclic ring structure was inspired by many medicinally relevant natural products that also contain the bi-aryl ether moiety. The synthesis required only commercially available starting materials and involved a base mediated SNAr cyclization. A conformational search was performed, which indicated a strong preference for a single conformation, which was consistent with observed ROE signals by NMR. 相似文献
992.
Modular chiral polyether podands and their lanthanide complexes 总被引:1,自引:0,他引:1
A novel series of modular chiral polyether podands derived from enantiomerically pure hydrobenzoin and binaphthol has been prepared using a NaH/15-crown-5 mediated Williamson ether synthesis. These new homochiral ligands form catalytically active complexes with lanthanide triflates, two of which have been characterised by X-ray diffraction. 相似文献
993.
Bhanu Nandan L. D. Kandpal G. N. Mathur 《Journal of Polymer Science.Polymer Physics》2004,42(8):1548-1563
Dynamic rheological measurements were carried out on blends of poly(ether ether ketone) (PEEK)/poly(aryl ether sulfone) (PES) in the melt state in the oscillatory shear mode. The data were analyzed for the fundamental rheological behavior to yield insight into the microstructure of PEEK/PES blends. A variation of complex viscosity with composition exhibited positive–negative deviations from the log‐additivity rule and was typical for a continuous‐discrete type of morphology with weak interaction among droplets. The point of transition showed that phase inversion takes place at composition with a 0.6 weight fraction of PEEK, which agreed with the actual morphology of these blends observed by scanning electron microscopy. Activation energy for flow, for blend compositions followed additive behavior, which indicated that PEEK/PES blends may have had some compatibility in the melt. Variation of the elastic modulus (G′) with composition showed a trend similar to that observed for complex viscosity. A three‐zone model used for understanding the dynamic moduli behavior of polymers demonstrated that PEEK follows plateau‐zone behavior, whereas PES exhibits only terminal‐zone behavior in the frequency range studied. The blends of these two polymers showed an intermediate behavior, and the crossover frequency shifted to the low‐frequency region as the PEEK content in PES increased. This revealed the shift of terminal‐zone behavior to low frequency with an increased PEEK percentage in the blend. Variation of relaxation time with composition suggested that slow relaxation of PEEK retards the relaxation process of PES as the PEEK concentration in the blend is increased because of the partial miscibility of the blend, which affects the constraint release process of pure components in the blend. A temperature‐independent correlation observed in the log–log plots of G′ versus loss modulus (G″) for different blend systems fulfilled the necessary condition for their rheological simplicity. Further, the composition‐dependent correlations of PEEK/PES blends observed in a log–log plot of G′ versus G″ showed that the blends are either partially miscible or immiscible and form a discrete‐continuous phase morphology. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1548–1563, 2004 相似文献
994.
995.
LiHuaJIA XiangFengGUO YuanYuanLIU XuHongQIAN 《中国化学快报》2004,15(1):118-120
A new benzoazacrown ether fluorescence sensor was synthesized with 9-anthrylmethyl chloride and benzoaza-15-crown-5 in CH3CN, which particularly shows a strong affinity for Zn^2 .Its fluorescence quantum yield increase more than one order of magnitude and a red shift could be noticed when passing from the apolar to the polar solvent. 相似文献
996.
用一种新合成的套索冠醚N,N′-双(乙酰苄胺)-二氮杂-18-冠-6作为固定相,涂渍在弹性石英毛细管内,测其柱效、惰性、热稳定性、平均极性及选择性等性质。实验表明,它具有良好的色谱性能,中等极性,分离选择性高,适用于对醇、卤代烃、芳香烃等各类异构体的分离,并从分子结构和热力学参数等探讨了保留机理。 相似文献
997.
We propose a new formulation for the æther of Dirac, which extends Dirac's original idea to a model where the vacuum is modified by the presence of the electromagnetic field. These results are then applied to the analysis of the amplification of the primordial magnetic induction in a curved background of Friedmann's geometry. 相似文献
998.
Jin Yan WANG Shu De XIAO Xi Gao JIAN* Department of Polymer Science & Materials Dalian University of Technology Dalian 《中国化学快报》2003,14(4)
Aromatic polyimides are distinguished for their excellent mechanical and thermal properties in many engineering fields1. However, aromatic polyimides are normally insoluble in common organic solvents, which restricts their applications in some fields. Many efforts have been taken to improve their solubility. Introducing bulky side groups and non-coplanar structure into polymers bone chains is a good way to obtain the polymers with excellent mechanical and thermal properties2. In this paper… 相似文献
999.
Sulfamic acid has been used as an efficient catalyst and green alternative for metal-containing acidic materials to promote the acetolysis reaction of THF to produce 1,4-diacetoxybutane. This method is also applicable to the acetolysis of other cyclic ethers, such as methyl substituted THF and tetrahydropyran and 1,4-dioxane, which is less reactive. 相似文献
1000.
Hermann Kalchhauser H. G. Krishnamurty A. C. Talukdar Walter Schmid 《Monatshefte für Chemie / Chemical Monthly》1988,119(8-9):1047-1050
Two new macrocyclic biaryl ethers have been isolated from the bark ofGaruga gamblei King (Burseraceae). The structure of the compounds has been established by NMR spectroscopy.
Isolierung und Strukturermittlung von zwei neuen makrocyclischen Biarylethern aus Garuga gamblei (Kurze Mitteilung)
Zusammenfassung Aus der Rinde vonGaruga gamblei King (Burseraceae) wurden zwei neue makrocyclische Biarylether isoliert. Die Struktur der Verbindungen wurde mittels NMR-Spektroskopie bestimmt.相似文献