全文获取类型
收费全文 | 4090篇 |
免费 | 219篇 |
国内免费 | 42篇 |
专业分类
化学 | 2668篇 |
晶体学 | 41篇 |
力学 | 132篇 |
数学 | 441篇 |
物理学 | 1069篇 |
出版年
2024年 | 16篇 |
2023年 | 38篇 |
2022年 | 69篇 |
2021年 | 218篇 |
2020年 | 170篇 |
2019年 | 168篇 |
2018年 | 170篇 |
2017年 | 136篇 |
2016年 | 207篇 |
2015年 | 147篇 |
2014年 | 212篇 |
2013年 | 376篇 |
2012年 | 308篇 |
2011年 | 302篇 |
2010年 | 237篇 |
2009年 | 190篇 |
2008年 | 211篇 |
2007年 | 182篇 |
2006年 | 137篇 |
2005年 | 74篇 |
2004年 | 99篇 |
2003年 | 56篇 |
2002年 | 57篇 |
2001年 | 32篇 |
2000年 | 28篇 |
1999年 | 32篇 |
1998年 | 22篇 |
1997年 | 29篇 |
1996年 | 26篇 |
1995年 | 12篇 |
1994年 | 26篇 |
1993年 | 20篇 |
1992年 | 21篇 |
1991年 | 11篇 |
1990年 | 19篇 |
1989年 | 13篇 |
1988年 | 15篇 |
1987年 | 16篇 |
1985年 | 23篇 |
1984年 | 23篇 |
1983年 | 15篇 |
1982年 | 20篇 |
1981年 | 13篇 |
1980年 | 37篇 |
1979年 | 11篇 |
1977年 | 15篇 |
1976年 | 10篇 |
1973年 | 11篇 |
1969年 | 9篇 |
1968年 | 8篇 |
排序方式: 共有4351条查询结果,搜索用时 31 毫秒
991.
Azobenzene-based calamitic liquid crystals, 4-((4-(4-methoxymethyl-1,2,3-triazol-1-yl)phenyl)diazenyl)phenylalkanoates have been isolated and their structures were characterised. The structure–property correlation with respect to the different alkanoyloxy terminal chain (–COOCnH2n+1 where n adopts odd numbers ranging from 3 to 15) has also been given attention in the present study. In this series, all compounds exhibit smectogenic properties. The lower homologues shows enantiotropic SmA phase as well as monotropic SmC phase. The higher homologues exhibit homeotropic alignment of smectic phases. These compounds possess very high anisotropic inclination in which the mesomorphic region covers nearly 80°C. 相似文献
992.
New bis(pyrazolylenaminones) were prepared in good yields. Their synthetic utilities as precursors for regioselective synthesis of novel bis(hetarylpyrazoles) were also investigated. The mechanisms and selectivities observed in the studied reactions were discussed. 相似文献
993.
T. Rüffer H. Lang S. Nawaz A. A. Isab S. Ahmad M. M. Athar 《Journal of Structural Chemistry》2011,52(5):1025-1029
A mixed-ligand silver(I) complex of triphenylphosphine and N,N′-dibutylthiourea (Dbtu), [Ag(Ph3P)2(Dbtu)2]NO3, is prepared and its structure in the solid state is determined by X-ray crystallography. X-ray structure of this complex
shows that it is mononuclear with the silver atom coordinated by two PPh3 and two dibutylthiourea ligands adopting a distorted tetrahedral geometry. The crystal structure shows the formation of 1-D
chains through intermolecular hydrogen bonding interactions between N-H of Dbtu and nitrate ions. The new complex is also
characterized by IR and NMR (1H and 31P) spectroscopy. The spectroscopic data are discussed in terms of the nature of bonding. A similar mixedligand complex is
also prepared for tetramethylthiourea (Tmtu), but the structure of the resulting compound shows that it is a bis(phosphine)
complex, [Ag(PPh3)2NO3] rather than a mixed-ligand complex. 相似文献
994.
Calculations [1] of rheological properties of polymeric solutions have usually been restricted to very dilute o r to very concentrated solutions. In either case, one starts with a model for the micro- structure. Between these two conceptual extremes lies the broad class of moderately concentrated solutions. To cover this region theoretically, two approaches have been used; 1) network theories focusing attention on entanglement junctions, and 2) single-molecule theories focusing attention on single molecules but including inter- molecular interactions. Williams, Wang, and Zimm introduced a double model; one for a particular macromolecule and another for solutions containing like molecules in which the chosen macro- molecule is suspended. This approach has turned out to be quite successful because spheres immersed in a suspension of like spheres behave as i f suspended in a Newtonian fluid, and the resulting ex- pression for viscosity is quite simple in form and qualitatively correct for moderate concentrations. 相似文献
995.
Kasim N. N. Mohamed A. R. Ishak M. A. M. Ahmad R. Nawawi W. I. Ali S. N. Ismail K. 《Journal of Thermal Analysis and Calorimetry》2019,137(1):343-350
Journal of Thermal Analysis and Calorimetry - The refined pitch (1#) and impregnating pitch (2#) were obtained under certain conditions with the modified pitch as the raw materials. The data of... 相似文献
996.
997.
998.
Esa Toukoniitty Päivi Mäki-Arvela Ahmad Kalantar Neyestanaki Ensio Laine Jyri-Pekka Mikkola Tapio Salmi Dmitry Yu. Murzin 《Reaction Kinetics and Catalysis Letters》2001,73(1):3-11
In the presence of ultrasound both the initial hydrogenation rate of 1-phenyl-1,2- propanedione and the enantiomeric excess of the main product, i.e. (R)-1-hydroxy- 1-phenylpropanone, increased by 75 % and 52 % in toluene, respectively, as compared to the silent hydrogenation. 相似文献
999.
Iraj Mohammadpoor-Baltork Ahmad R. Khosropour Seyedeh F. Hojati 《Monatshefte für Chemie / Chemical Monthly》2007,138(7):663-667
Summary. A series of benzoxazoles, benzothiazoles, benzimidazoles, and oxazolo[4,5-b]pyridines was efficiently synthesized from the reactions of o-aminophenols, o-aminothiophenol, o-phenylenediamines, and 2-amino-3-hydroxypyridine with orthoesters in the presence of catalytic amounts of Bi(III) salts,
such as Bi(TFA)3, Bi(OTf)3, and BiOClO4 · xH2O under solvent-free conditions. The remarkable features of this new protocol are high conversion, very short reaction times,
cleaner reaction profiles under solvent-free conditions, straightforward procedure, and use of relatively non-toxic catalysts. 相似文献
1000.