全文获取类型
收费全文 | 112381篇 |
免费 | 19188篇 |
国内免费 | 11178篇 |
专业分类
化学 | 76315篇 |
晶体学 | 1325篇 |
力学 | 7246篇 |
综合类 | 637篇 |
数学 | 12867篇 |
物理学 | 44357篇 |
出版年
2024年 | 391篇 |
2023年 | 2226篇 |
2022年 | 3869篇 |
2021年 | 4099篇 |
2020年 | 4489篇 |
2019年 | 4404篇 |
2018年 | 3763篇 |
2017年 | 3507篇 |
2016年 | 5491篇 |
2015年 | 5317篇 |
2014年 | 6451篇 |
2013年 | 8317篇 |
2012年 | 10081篇 |
2011年 | 10394篇 |
2010年 | 6941篇 |
2009年 | 6678篇 |
2008年 | 7114篇 |
2007年 | 6488篇 |
2006年 | 5925篇 |
2005年 | 4940篇 |
2004年 | 3709篇 |
2003年 | 2891篇 |
2002年 | 2625篇 |
2001年 | 2160篇 |
2000年 | 1878篇 |
1999年 | 2192篇 |
1998年 | 1944篇 |
1997年 | 1801篇 |
1996年 | 1982篇 |
1995年 | 1587篇 |
1994年 | 1521篇 |
1993年 | 1224篇 |
1992年 | 1116篇 |
1991年 | 1021篇 |
1990年 | 818篇 |
1989年 | 586篇 |
1988年 | 492篇 |
1987年 | 401篇 |
1986年 | 389篇 |
1985年 | 339篇 |
1984年 | 250篇 |
1983年 | 163篇 |
1982年 | 145篇 |
1981年 | 111篇 |
1980年 | 78篇 |
1979年 | 45篇 |
1978年 | 34篇 |
1976年 | 36篇 |
1975年 | 33篇 |
1974年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
6-Oxy-(N-succinimidyl acetate)-9-(2'-methoxycarbonyl)fluorescein (SAMF), a new fluorescein-based amine-reactive fluorescent probe was well designed, synthesized and used as a pre-column derivatizing reagent for the determination of aliphatic amines in HPLC. It exhibited relatively pH-independent fluorescence (pH 4-9) and excellent photostability. The derivatization was performed at room temperature in 6min. On a C18 column, the derivatives of SAMF with eight aliphatic amines were baseline separated in 28 min with a mobile phase of methanol-water (57:43, v/v) containing 10 mmol l(-1) pH 5.0, H3Cit3-NaOH buffer. With fluorescent detection at lambda(ex)/lambda(em) = 484/516 nm, the detection limit could reach 2-320 fmol (signal-to-noise = 3), which was equivalent to or better than the detection limits obtained from other analytical methods of aliphatic amines. The proposed method has been applied to the determination of the aliphatic amines in environmental and food samples such as lake water, red wine, white wine, and cheese with satisfying recoveries varying from 95 to 106%. 相似文献
982.
The synthesis of a fluorous diol 4 bearing a perfluorodecyl chain was described. A series of boronic acid were attached to 4 by esterification. The purification of the products was fulfilled by facile filtration instead of expensive and environmental troublesome fluorous liquid-liquid extraction. The Suzuki cross-coupling reactions of the formed fluorous boronates 5 underwent smoothly and the fluorous diol 4 was recycled in good yields. 相似文献
983.
The interactions of nucleic acids and cationic surfactants (cetylpyridine bromide (CPB) and cetyltrimethylammonium bromide (CTMAB)) in aqueous solution have been studied using the techniques of resonance light scattering (RLS) spectroscopy, the absorption spectroscopy, zeta potential assay and NMR assignment measurement. It is considered that CPB or CTMAB can assemble on the surface of nucleic acid via electrostatic and hydrophobic forces, which results in the formation of large associate of nucleic acid-cationic surfactant and RLS enhancement of nucleic acid. Besides these forces, the pi-pi stacking force between CPB and nucleic acid also exists in the associate. In comparison with CTMAB, CPB has larger enhancement on RLS of nucleic acid, which is attributed to that the enhancement of the former is only due to the absorption of the bases of nucleic acid, while the enhancement of the latter is own to the synergetic resonance caused by the absorption of both bases of nucleic acid and the pyridyl in CPB. These results have important implication for understanding the influence of surfactants on nucleic acid functionality in life science. 相似文献
984.
A brief overview of our recent research results of thermoregulated liquid/liquid biphasic catalysis is presented. Emphasis is given to the general principles of thermoregulated phase-transfer catalysis (TRPTC) and thermoregulated phase-separable catalysis (TPSC). In addition, the applications of TRPTC and TPSC in biphasic catalysis are also discussed. The introduction of TRPTC to biphasic system is free from the shortcomings of classical aqueous/organic two-phase catalysis, in which the application scope is restrained by the water solubility of the substrate. Meanwhile, TPSC provides a very simple and reliable way to deal with the separation of catalyst in homogeneous catalysis. The common advantages of TRPTC and TPSC are characterized by homogeneous catalysis coupled with convenient biphasic separation. 相似文献
985.
Tricalix[4]arene 4, tricalix[5]arene 14, and pentacalix[4]arene 10 have been synthesized from O-alkylcalixarene mono- and dialdehydes by a two-step conversion to the corresponding monoethynyl ketones or diethynyl ketones followed by aryne trimerization in refluxing DMF containing a dialkylamine. The tricalixarenes 4 and 14 were converted, in turn, to calixarenes 6 and 16, which carry OH groups on the lower rim and methylenes as the bridging moieties to the benzene ring. Complexation studies with the tricalix[5]arene 16 show that it forms (a) a 1:3 complex with N,N-dimethylethylenediamine in which each calixarene unit contains a molecule of the amine, (b) a 1:1 complex with tris(aminomethyl)amine in which each calixarene unit contains one of the three arms of the guest, and (c) a 1:1 complex with C60 in which the guest presumably resides in the cavity provided by the three calixarene units acting cooperatively. 相似文献
986.
987.
用射央求地在硅单晶衬底上沉积出电导率高达60S/cm的a-SiHY合金薄膜。在20-300K,对于钇含量高的样品,其电导仍是热激活的。lgσ与1/T的关系曲线能够被拟合成斜率不同的两条直线,直线的斜率和两直线间拐点所地 温度依赖于膜中钇的含量。但对钇含量低的薄膜,电导对温度的依赖关系度为σ∝exp(-1/T^1/4)。结果表明,这些钇含量不同的样品在没的温度范围内具有不同的导电机制。 相似文献
988.
Enantiopure (+)-2,5-dimethylthiolane and (-)-2,5-dimethylsulfolane were prepared using literature procedures and investigated using vibrational circular dichroism (VCD). Experimental absorption and VCD spectra of (+)-2,5-dimethylthiolane and (-)-2,5-dimethylsulfolane in CCl(4) solution in the 2000-900 cm(-)(1) region were compared with the ab initio predictions of absorption and VCD spectra obtained with density functional theory using the B3LYP/6-311G(2d, 2p) basis set for different conformers of (2R,5R)-2,5-dimethylthiolane and (2R,5R)-2,5-dimethylsulfolane. This comparison indicates that (+)-2,5-dimethylthiolane is of the (2R,5R)-configuration and has two predominant conformations in CCl(4) solution. In addition, (-)-2,5-dimethylsulfolane is of (2R,5R)-configuration and has only one predominant conformation. The stereochemical assignment is in agreement with literature. 相似文献
989.
测定了Pt-Sn型催化剂浸渍状态下的Sn-119、Pt-195的多核核磁共振。当SnCl2/DCl溶液体系中加入H2PtCl6以后,出现了Sn(Ⅳ)和另外一种Sn(Ⅱ)的构型,Sn-119峰向高场位移,说明部分Sn(Ⅱ)被氧化成Sn(Ⅳ),H2PtCl6的量对这种氧化性影响较小。而H2PtCl6/D2O溶液体系中加入SnCl2以后部分Pt(Ⅳ)被还原成Pt(Ⅱ),随着SnCl2量的增加,Pt(Ⅱ) 相似文献
990.
Study on the Phase Diagram of CsCl-CeCl3-HCl(11%)-H2O System and the Properties of the Compounds 总被引:2,自引:0,他引:2
The equilibrium solubility of CsCl-CeCl3-HCl(11%)-H2O qua-ternary system at 25℃ has been determined by the physicchemical analysis method ,and the phase diagram was plotted, Two new double salts 3CsCl.CeCl3.3H2O and CsCl.CeCl3.4H2O obtained from the complicated system were identified and characterized by XRD,TG-DTA ,DSC,UV and fluorescence spectroscopy, Studies on the fluorescence excitation and emission show that 3CsCl.CeCl3.3H2O and CsCl.CeCl3.4H2O have upconversion luminescence of infrared-visible range,and the upconversion emission intensity increases with the increase of ratio of CeCl3 in CsCl. 相似文献