全文获取类型
收费全文 | 5557篇 |
免费 | 124篇 |
国内免费 | 1035篇 |
专业分类
化学 | 6017篇 |
晶体学 | 89篇 |
力学 | 24篇 |
综合类 | 34篇 |
数学 | 8篇 |
物理学 | 544篇 |
出版年
2024年 | 5篇 |
2023年 | 51篇 |
2022年 | 77篇 |
2021年 | 82篇 |
2020年 | 105篇 |
2019年 | 89篇 |
2018年 | 122篇 |
2017年 | 137篇 |
2016年 | 141篇 |
2015年 | 113篇 |
2014年 | 210篇 |
2013年 | 522篇 |
2012年 | 282篇 |
2011年 | 301篇 |
2010年 | 254篇 |
2009年 | 317篇 |
2008年 | 337篇 |
2007年 | 392篇 |
2006年 | 367篇 |
2005年 | 371篇 |
2004年 | 355篇 |
2003年 | 277篇 |
2002年 | 215篇 |
2001年 | 167篇 |
2000年 | 158篇 |
1999年 | 155篇 |
1998年 | 111篇 |
1997年 | 110篇 |
1996年 | 124篇 |
1995年 | 128篇 |
1994年 | 117篇 |
1993年 | 108篇 |
1992年 | 92篇 |
1991年 | 47篇 |
1990年 | 39篇 |
1989年 | 46篇 |
1988年 | 36篇 |
1987年 | 13篇 |
1986年 | 29篇 |
1985年 | 12篇 |
1984年 | 8篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 14篇 |
1980年 | 17篇 |
1979年 | 12篇 |
1977年 | 6篇 |
1976年 | 4篇 |
1974年 | 4篇 |
1972年 | 4篇 |
排序方式: 共有6716条查询结果,搜索用时 15 毫秒
51.
A new salicylic-based open-chain crown ether ligand, 1,10-bis(2′-carboxylphenyl)-1,4,7,10-tetraoxadecane (BCPTD) was synthesized. Solutions of its complex with Tb3+ can emit the intrinsic fluorescence of Tb3+. The fluorescence intensity of the complex in KCl solution was enhanced by the addition of silver(I), leading to a new fluorescence enhancement phenomenon. The spectrofluorimetric determination of traces of silver(I) based on the above phenomenon was carried out. The excitation and emission wavelengths are 298 and 545 nm, respectively. Under optimal conditions, the differential value of fluorescence intensity in the absence and presence of Ag+ was proportional to the concentration of silver(I) in the range 0.5-20 μg ml−1. The method was applied to the determination of silver(I) in a standard ore sample. The analytical performance is investigated in detail by using common aromatic carboxylic acids or synthetic analogues of BCPTD as ligands to replace BCPTD. It was found that Tb-aromatic acid complexes did not result in fluorescence enhancement of Tb3+ in AgCl collosol. The phenomenon was only observed in Tb(III) with BCPTD or its open-chain crown ether analogues solutions.In addition, the enhancement of the fluorescence intensity of terbium(III) in these complexes depends on the extent of formation of the AgCl collosol. 相似文献
52.
When promoted by samarium in DMF, aroyl chlorides react readily with acrylates to afford α,β-diaroylpropionates in good to excellent yields without pretreating or activating the metallic samarium. 相似文献
53.
María?Belén?Sierra Marcela?A.?Morini Pablo?C.?SchulzEmail author María?Luján?Ferreira 《Colloid and polymer science》2005,283(9):1016-1024
Following the studies on the effect of double bonds in the surfactant hydrophobic tail on the formation of mixed surfactant aggregates, we studied the viscosity and density of the system Sodium 10-undecenoate (SUD)–decyltrimethylammonium bromide (DTAB)–water. We found that the partial molar volume (pmv) and intrinsic viscosity of both, micellised and unmicellised mixtures, are non-ideal, dependent on the mixture composition and related to structural changes in micelles. These phenomena are caused by the presence of the double bond at the distal extreme of the SUD molecule, which has some affinity with water by formation of hydrogen bonds. In particular, as far as we know, this is the first report on non-ideal behavior of the pmv in mixed micelles. 相似文献
54.
The aqueous cationic system sodium undecenoate-dodecyltrimethylammonium bromide at low concentration
Maria?Belén?Sierra Marcela?A.?Morini Pablo?C.?SchulzEmail author 《Colloid and polymer science》2004,282(6):633-641
The aqueous sodium undecenoate (SUD) –dodecyltrimethylammonium bromide (DTAB) catanionic system was studied at low concentration. The system did not precipitate, even at a 1:1 SUD:DTAB proportion, but showed the formation of a coacervate in a range of surfactant mixture compositions. Micelles have a preferential composition of 0.37 mole fraction of SUD. This behavior is attributed to the presence of the double bond at the distal extreme of the SUD molecule, which can form hydrogen bonds with water. Consequently, the –CH=CH2 group is situated at the interface between the hydrocarbon micelle core and water, reducing the interfacial free energy. Structural computations demonstrate that the mentioned SUD proportion produces complete coverage of the micelle surface by the double bonds. 相似文献
55.
56.
氯化1-乙基-3-甲基烷基咪唑分子结构及其氢键作用能的密度泛函研究 总被引:1,自引:0,他引:1
采用量子化学密度范函方法计算研究了氯化1-乙基-3-甲基咪唑盐([EMIM]C1)正负离子间相互作用的平衡构型和Cl^-离子从不同方位与咪唑阳离子的结合能.计算发现Cl^-可在咪唑环平面上结合形成离子键,其结合能为.368.97kJ/mol,还可与环上的三个H原子形成氢键,其氢键键长和结合能分别为0.2000nm/-378.03kJ/mol、0.2339nm/-344,69kJ/mol和0.2301nm/-346,01kJ/mol.结合能包括氢键作用能和正负离子的静电作用能,后者为主要贡献.从理论上展示了该离子液体的氢键结构. 相似文献
57.
气相色谱法测定三聚氰氯的研究李秀梅赵树岚*田锦霞*孙维星(东北师范大学化学系长春130024)关键词气相色谱法三聚氰氯三聚氰氯(CNCl)3是活性染料的重要中间体及合成树脂、橡胶、塑料等的防老剂,也常用于除草剂、增白剂、药物、炸药等的生产中。传统的分... 相似文献
58.
Hidetoshi Taima 《Tetrahedron letters》2005,46(24):4161-4164
Two esters of chlorin e6 have been synthesized by esterification with aminoalcohols via the formation of acid chloride using oxalyl chloride and converted to the corresponding cationic water-soluble esters of chlorin e6. The visible absorption and circular dichroism spectra have revealed that these two cationic chlorin e6 esters synthesized are readily soluble in water as a monomer. 相似文献
59.
Electromotive force measurements were carried out on the system KCl–KNO3–H2O at constant total ionic strengths of 0.5, 1.0, 2.0 and 3.0 mol-kg–1 and at 25, 35 and 45°C using a cell consisting of a potassium ionselective electrode and a Ag/AgCl electrode. The Harned coefficients and the Pitzer binary and ternary interaction parameters for the system have been evaluated at each temperature. The osmotic coefficients, excess free energies of mixing and heats of mixing of the system have been predicted at each of the experimental temperatures and ionic strengths. The solubility data at 25°C are also interpreted. 相似文献
60.
A background-free, fast protein staining method in polyacrylamide gel electrophoresis using an acidic dye, zincon (ZC) and a basic dye, ethyl violet (EV) is described. It is based on the counterion dye staining technique that employs two oppositely charged dyes to form an ion-pair complex in staining solution. The selective binding of free dye molecules to proteins in acidic solution produces bluish violet-colored bands. It is a rapid and end-point staining procedure, involving only fixing and staining steps that are completed in 1-1.5 h. The detection limit of this method is 8-15 ng of protein that is comparable to the sensitivity of the colloidal Coomassie Brilliant Blue G (CBBG) stain. Due to its sensitivity and speed, this stain may be more practical than any other dye-based stains for routine laboratory purposes. 相似文献