全文获取类型
收费全文 | 3192篇 |
免费 | 479篇 |
国内免费 | 432篇 |
专业分类
化学 | 2327篇 |
晶体学 | 30篇 |
力学 | 225篇 |
综合类 | 46篇 |
数学 | 314篇 |
物理学 | 1161篇 |
出版年
2024年 | 8篇 |
2023年 | 57篇 |
2022年 | 110篇 |
2021年 | 115篇 |
2020年 | 144篇 |
2019年 | 109篇 |
2018年 | 118篇 |
2017年 | 117篇 |
2016年 | 149篇 |
2015年 | 130篇 |
2014年 | 177篇 |
2013年 | 219篇 |
2012年 | 214篇 |
2011年 | 292篇 |
2010年 | 201篇 |
2009年 | 196篇 |
2008年 | 208篇 |
2007年 | 192篇 |
2006年 | 158篇 |
2005年 | 121篇 |
2004年 | 95篇 |
2003年 | 75篇 |
2002年 | 75篇 |
2001年 | 69篇 |
2000年 | 60篇 |
1999年 | 73篇 |
1998年 | 60篇 |
1997年 | 50篇 |
1996年 | 48篇 |
1995年 | 51篇 |
1994年 | 51篇 |
1993年 | 48篇 |
1992年 | 47篇 |
1991年 | 37篇 |
1990年 | 40篇 |
1989年 | 32篇 |
1988年 | 25篇 |
1987年 | 31篇 |
1986年 | 24篇 |
1985年 | 13篇 |
1984年 | 8篇 |
1983年 | 11篇 |
1982年 | 12篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1957年 | 4篇 |
排序方式: 共有4103条查询结果,搜索用时 15 毫秒
151.
A new one-step synthesis of 3-hydroxymethylbenzofuran, based on intramolecular cyclization of 2- (methoxymethyl)-2-(2'-methoxymethyl-4'-methylphenyl)-butanone I under diluted hydrochloric acid in THF, was developed. The mechanism for this process was investigated via chemical equilibrium shift of tautomer in acidic conditions. The applicability of this new method was studied further in this paper. 相似文献
152.
153.
154.
Jun Ni Yi‐Zhi Li Wen‐Bin Qi Yong‐Jiang Liu Hui‐Lan Chen Zhi‐Lin Wang 《Acta Crystallographica. Section C, Structural Chemistry》2003,59(8):o470-o472
The title compound, 3,3′‐(4‐pyridylimino)dipropanenitrile, C11H12N4, has a twofold axis and consists of a pyridine ring head and two cyanoethyl tails, the three groups being linked by an N atom. The planar geometry around the amino N atom suggests conjugation with the π‐system of the pyridine ring. The molecules are stacked in a layer structure via relatively weak to very weak intermolecular C—H⃛π and C—H⃛N hydrogen‐bond interactions. 相似文献
155.
Lingjuan Xiong Rong Wang Chen Liang Fangqi Cao Yulan Rao Xin Wang Libo Zeng Chunfang Ni Haiying Ye Yurong Zhang 《Analytical and bioanalytical chemistry》2013,405(30):9805-9816
Ecgonine is suggested to be a promising marker of cocaine (COC) ingestion. A combined mass spectrometry (MS) and tandem MS (MS/MS) method was developed to simultaneously determine ecgonine and seven other metabolites of cocaine in human urine and whole blood with ultra-high-pressure liquid chromatography coupled with quadrupole time-of-flight mass spectrometry. The compounds were extracted from as little as 100 μL of sample by solid-phase extraction with a 96-well μElution solid-phase extraction plate. The protonated molecules or fragment ions at accurate mass acquired in MS mode were used to quantify specific analytes, following by dedicated MS/MS identification. The assay was linear in the range from 5 to 50-100 ng/mL for urine samples, except for ecgonine methyl ester (10-200 ng/mL) and ecgonine (40-400 ng/mL), and was linear from 1-2 to 50 ng/mL for whole blood samples, except for ecgonine methyl ester (20-1,000 ng/mL) and ecgonine (40-2,000 ng/mL). The correlation coefficients were all greater than 0.99. The limits of detection ranged from 0.2 to 16 ng/mL, and the lower limits of quantification ranged from 1 to 40 ng/mL. The repeatability and intermediate precision were 18.1 % or less. The accuracy was in the range from 80.0 to 122.9 %, process efficiencies were in the range from 8.6 to 177.4 %, matrix effects were in the range from 28.7 to 171.0 %, and extraction recoveries were in the range from 41.0 to 114.3 %, except for ecgonine (12.8 % and 9.3 % at low and high concentrations, respectively). This method was highly sensitive in comparison with previously published methods. The validated method was successfully applied to the analysis of real samples derived from forensic cases, and the results verified that, on the basis of data from four positive samples, ecgonine is a promising marker of cocaine ingestion. Figure
Procedure for the determination of ecgonine and seven other cocaine metabolites in human urine and whole blood using a combined mass spectrometry and tandem MS method aIer the solid‐phase extraction of the anaytes 相似文献
156.
157.
Haihua Wang Naravit Leaukosol Zhibing He Guiqiang Fei Chuanling Si Yonghao Ni 《Cellulose (London, England)》2013,20(4):1587-1601
Highly intrinsic conductive polypyrrole/cellulose fiber composites (CF) were successfully prepared through in situ chemical oxidation polymerization simply by increasing fiber concentration at the same dosage of pyrrole, oxidant and dopant (based on the weight of dry fiber). FeCl3 and anthraquinone-2-sulfonic acid sodium salt (AQSNa) were utilized as oxidant and dopant. As fiber concentration increased from 1 % (CF1) to 20 % (CF20), N and S content increased from 0.24 and 0.25 % to 1.24 and 0.89 %, and great increase in the retention of PPy and AQSNa was confirmed by elemental analysis. In addition, on the surface of conductive fiber, PPy of compact fibroid structure was detected instead of interconnected globular structure at higher fiber concentration. Furthermore, scanning transmission electron microscope and X-ray photoelectron spectroscopy (XPS)-depth profile analysis demonstrated denser and more uniformly distributed PPy inside fiber wall for CF20, while PPy tended to deposit on the surface of fiber for CF1. Fourier transform infrared spectroscopy, together with XPS certified that the PPy with longer conjugation length and higher doping level across the conductive fiber was obtained at higher fiber concentration. The doping level for CF10 decreased from 21.55 to 16.39 % with increasing fiber wall thickness, while that of CF20 decreased slightly from 30.73 to 24.10 %. The resulting CF20 showed lowest surface resistivity of 0.433 KΩ/square, as well as improved electro-conductivity stability. The incorporation of more PPy in CF improved the thermal stability. 相似文献
158.
(Z)-γ-Cyano-β-perfluoroalkyl-β,γ-unsaturated esters have been synthesized by the reaction of perfluoroacylated phosphonates with organozinc reagent in 50-71% (three steps) yields. 相似文献
159.
Zhang WW Ren XM Li H Xie JL Lu CS Zou Y Ni ZP Meng QJ 《Journal of colloid and interface science》2003,268(1):173-180
Self-assembled monolayers (SAMs) of functionalized azobenzene thiols (RAzoCnSH, n=3-6 for R=H, abbreviated as AzoCnSH; and n=4 for R=CH(3)CONH, abbreviated as aaAzoC4SH) on different substrates RAzoCnSz.sbnd;z.sfnc;S (S represents substrates of vacuum-deposited gold (Au), silver foil (Ag), HNO(3) etched silver foil (EAg), and silver mirror (mAg)) have been studied by SERS in the near-infrared region. SERS of the SAMs on EAg and/or mAg exhibit SERS effects that vary with etching time and/or deposition time. The most appropriate time is 5 s for etching in 1:1 HNO(3) and 40 s for deposition in 0.1 M Ag(NH(3))(2)NO(3). Further, a layer of Ag mirror was conveniently deposited on the top of the SAMs on different substrates, yielding a more efficient SERS-active system possessing a "sandwiched" structure of mAgz.sfnc;RAzoCnS-z.sfnc;S. An appropriate surface roughness is required for the strongest SERS effect. Scanning electron microscopy (SEM) indicates that there exist a large number of projects around 100 nm on the surface showing the strongest SERS effect. When the surface roughness is decreased or increased, the SERS effect decreases sharply. The relationship between the SERS effect and the structural nature was investigated and showed that the enhancement factor decays exponentially with increasing in distances of the azobenzene group from the underlying substrate or the overlying silver mirror. This result reveals that the SERS effect may be the result of the electromagnetic coupling effect between two metal layers. 相似文献
160.
Rodríguez Patino JM Molina Ortiz SE Carrera Sánchez C Rodríguez Niño MR Añón MC 《Journal of colloid and interface science》2003,268(1):50-57
In this paper we present surface dilatational properties of soy globulins (beta-conglycinin, glycinin, and reduced glycinin with 10 mM of dithiothreitol (DTT)) adsorbed onto the air-water interface, as a function of adsorption time. The experiments were performed at constant temperature (20 degrees C), pH (8.0), and ionic strength (0.05 M). The surface rheological parameters were measured as a function of protein concentration (ranging from 1 to 1x10(-3)% wt/wt). We found that the surface dilatational modulus, E, increases, and the phase angle, phi, decreases with time, theta, which may be associated with protein adsorption. These phenomena have been related to protein adsorption, unfolding, and/or protein-protein interactions (at long-term adsorption) as a function of protein concentration in solution. From a rheological point of view, the surface viscoelastic characteristics of soy globulin films adsorbed at the air-water interface are practically elastic. The main conclusion is that the dilatational properties of the adsorbed films depend on the molecular structure of the protein. 相似文献