全文获取类型
收费全文 | 75771篇 |
免费 | 5360篇 |
国内免费 | 2588篇 |
专业分类
化学 | 49351篇 |
晶体学 | 513篇 |
力学 | 3358篇 |
综合类 | 221篇 |
数学 | 10762篇 |
物理学 | 19514篇 |
出版年
2023年 | 835篇 |
2022年 | 1161篇 |
2021年 | 1437篇 |
2020年 | 1673篇 |
2019年 | 1598篇 |
2018年 | 1829篇 |
2017年 | 1533篇 |
2016年 | 2678篇 |
2015年 | 2585篇 |
2014年 | 2764篇 |
2013年 | 4826篇 |
2012年 | 5516篇 |
2011年 | 5777篇 |
2010年 | 3546篇 |
2009年 | 3126篇 |
2008年 | 4449篇 |
2007年 | 4342篇 |
2006年 | 3969篇 |
2005年 | 3665篇 |
2004年 | 3181篇 |
2003年 | 2557篇 |
2002年 | 2260篇 |
2001年 | 1549篇 |
2000年 | 1338篇 |
1999年 | 1069篇 |
1998年 | 846篇 |
1997年 | 746篇 |
1996年 | 866篇 |
1995年 | 664篇 |
1994年 | 726篇 |
1993年 | 644篇 |
1992年 | 697篇 |
1991年 | 586篇 |
1990年 | 547篇 |
1989年 | 503篇 |
1988年 | 380篇 |
1987年 | 377篇 |
1986年 | 334篇 |
1985年 | 504篇 |
1984年 | 444篇 |
1983年 | 315篇 |
1982年 | 420篇 |
1981年 | 381篇 |
1980年 | 342篇 |
1979年 | 323篇 |
1978年 | 304篇 |
1976年 | 305篇 |
1975年 | 280篇 |
1974年 | 274篇 |
1973年 | 278篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
Synthesis of partially fluorinated poly(arylene ether sulfone) multiblock copolymers bearing perfluorosulfonic functions 下载免费PDF全文
Luca Assumma Cristina Iojoiu Régis Mercier Sandrine Lyonnard Huu Dat Nguyen Emilie Planes 《Journal of polymer science. Part A, Polymer chemistry》2015,53(16):1941-1956
Partially fluorinated poly(arylene ether sulfone) multiblock copolymers bearing perfluorosulfonic functions (ps‐PES‐FPES), with ionic exchange capacity (IEC) ranging between 0.9 and 1.5 meq H+/g, are synthesized by regioselective bromination of partially fluorinated poly(arylene ether sulfone) multiblock copolymers (PES‐FPES), followed by Ullman coupling reaction with lithium 1,1,2,2‐tetrafluoro‐2‐(1,1,2,2‐tetrafluoro‐2‐iodoethoxy)ethanesulfonate. The PES‐FPES are prepared by aromatic nucleophilic substitution reaction by an original approach, that is, “one pot two reactions synthesis.” The chemical structures of polymers are analyzed by 1H and 19F NMR spectroscopy. The resulted ionomers present two distinct glass transitions and α relaxations revealing phase separation between the hydrophilic and the hydrophobic domains. The phase separation is observed at much lower block lengths of ps‐PES‐FPES as compared with the literature. AFM and SANS observations supported the phase separation, the hydrophilic domains are well dispersed but the connectivity to each other depends on the ps‐PES block lengths. The thermomechanical behavior, the water up‐take, and the conductivity of the ps‐PES‐FPES membranes are compared with those of Nafion 117® and randomly functionalized polysulfone (ps‐PES). Conductivities close or higher to those of Nafion 117® are obtained. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1941–1956 相似文献
12.
13.
14.
15.
We propose a system of time-division multiplexing(TDM) and spatial frequency-division multiplexing(SFDM). Extrinsic Fabry–Perot interferometric sensors are applied to detect weak acoustic signals. The broadband source is employed, the light from it is modulated by a pulse signal sequence and is efficiently amplified by semiconductor optical amplifiers. Experimental results show that the equivalent noise pressure spectrum level is-97.2 d B re 1 rad/√Hz below 1250 Hz, and the cross talk between two sensors in one TDM channel is-32.7 d B with a cavity length difference of 60 μm. The number of sensors in this multiplexing system can theoretically reach 160. 相似文献
16.
17.
18.
Chris H. Hill Agnete H. Viuff Samantha J. Spratley Stéphane Salamone Stig H. Christensen Randy J. Read Nigel W. Moriarty Henrik H. Jensen Janet E. Deane 《Chemical science》2015,6(5):3075-3086
Krabbe disease is a devastating neurodegenerative disorder characterized by rapid demyelination of nerve fibers. This disease is caused by defects in the lysosomal enzyme β-galactocerebrosidase (GALC), which hydrolyzes the terminal galactose from glycosphingolipids. These lipids are essential components of eukaryotic cell membranes: substrates of GALC include galactocerebroside, the primary lipid component of myelin, and psychosine, a cytotoxic metabolite. Mutations of GALC that cause misfolding of the protein may be responsive to pharmacological chaperone therapy (PCT), whereby small molecules are used to stabilize these mutant proteins, thus correcting trafficking defects and increasing residual catabolic activity in cells. Here we describe a new approach for the synthesis of galacto-configured azasugars and the characterization of their interaction with GALC using biophysical, biochemical and crystallographic methods. We identify that the global stabilization of GALC conferred by azasugar derivatives, measured by fluorescence-based thermal shift assays, is directly related to their binding affinity, measured by enzyme inhibition. X-ray crystal structures of these molecules bound in the GALC active site reveal which residues participate in stabilizing interactions, show how potency is achieved and illustrate the penalties of aza/iminosugar ring distortion. The structure–activity relationships described here identify the key physical properties required of pharmacological chaperones for Krabbe disease and highlight the potential of azasugars as stabilizing agents for future enzyme replacement therapies. This work lays the foundation for new drug-based treatments of Krabbe disease. 相似文献
19.
Ozonolysis of a Series of Methylated Alkenes: Reaction Rate Coefficients and Gas‐Phase Products 下载免费PDF全文
Tristan Braure Véronique Riffault Alexandre Tomas Romeo Iulian Olariu Cecilia Arsene Yuri Bedjanian Patrice Coddeville 《国际化学动力学杂志》2015,47(9):596-605
The gas‐phase ozonolysis of three methylated alkenes, i.e., trans‐2,2‐dimethyl‐3‐hexene (22dM3H), trans‐2,5‐dimethyl‐3‐hexene (25dM3H), and 4‐methyl‐1‐pentene (4M1P), has been investigated in the presence of sufficient hydroxyl radical scavenger in a laminar flow reactor at ambient temperature (296 ± 2 K) and P = 1 atm of dry air (RH ≤ 5%). Ozone levels in the reactor were monitored by an automatic analyzer. Alkene and gas‐phase product concentrations were determined via online sampling either on three‐bed adsorbent cartridges followed by thermodesorption and GC/FID‐MS analysis or on 2,4‐dinitrophenylhydrazine (DNPH) cartridges for subsequent HPLC/UV analysis. Reaction rate coefficients of (3.38 ± 0.12) × 10?17 for 22dM3H and (2.71 ± 0.26) × 10?17 for 25dM3H, both in cm3 molecule?1 s?1 units, have been obtained under pseudo–first‐order conditions. Primary carbonyl products have been identified for the three investigated alkenes, and branching ratios are reported. In the case of 4M1P ozonolysis, the yield of a Criegee intermediate was indirectly determined. Kinetics and product study results are compared to those of literature when available. This work represents the first investigation of reaction products in the ozonolysis of 22dM3H, 25dM3H, and 4M1P in a flow reactor. 相似文献
20.