全文获取类型
收费全文 | 103111篇 |
免费 | 4872篇 |
国内免费 | 3216篇 |
专业分类
化学 | 41291篇 |
晶体学 | 1032篇 |
力学 | 8452篇 |
综合类 | 218篇 |
数学 | 35046篇 |
物理学 | 25160篇 |
出版年
2024年 | 65篇 |
2023年 | 572篇 |
2022年 | 682篇 |
2021年 | 912篇 |
2020年 | 1093篇 |
2019年 | 1009篇 |
2018年 | 11115篇 |
2017年 | 10865篇 |
2016年 | 7215篇 |
2015年 | 2035篇 |
2014年 | 1753篇 |
2013年 | 2271篇 |
2012年 | 6084篇 |
2011年 | 12778篇 |
2010年 | 7170篇 |
2009年 | 7520篇 |
2008年 | 8230篇 |
2007年 | 10117篇 |
2006年 | 1571篇 |
2005年 | 2433篇 |
2004年 | 2354篇 |
2003年 | 2619篇 |
2002年 | 1612篇 |
2001年 | 759篇 |
2000年 | 809篇 |
1999年 | 780篇 |
1998年 | 718篇 |
1997年 | 665篇 |
1996年 | 683篇 |
1995年 | 582篇 |
1994年 | 462篇 |
1993年 | 437篇 |
1992年 | 338篇 |
1991年 | 314篇 |
1990年 | 286篇 |
1989年 | 242篇 |
1988年 | 203篇 |
1987年 | 183篇 |
1986年 | 149篇 |
1985年 | 140篇 |
1984年 | 104篇 |
1983年 | 86篇 |
1982年 | 93篇 |
1981年 | 60篇 |
1980年 | 58篇 |
1979年 | 46篇 |
1914年 | 45篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 890 毫秒
991.
The separation of enantiomers and diastereomers of 8 commonly used pesticides was investigated by liquid chromatography (LC) using a Chiralcel OD column (cellulose tris-3,5-dimethylphenylcarbamate as the chiral stationary phase) and a Pirkle-type Chirex 3020 column (urea derivative from the reaction of (R)-1-(alpha-naphthyl)ethylamine with (S)-tert-leucine, chemically bonded to 3-aminopropylsilanized silica as the chiral stationary phase). The pesticides studied included one organophosphorus insecticide (phenthoate), 3 triazole fungicides (uniconazole, diniconazole, and propiconazole), and 4 pyrethroids (fenpropathrin, beta-cypermethrin, beta-cyfluthrin, and alpha-fenvalerate). The enantiomers were separated within 20 min with a resolution of > or = 1.5 using a mixture of n-hexane and 2-propanol as the mobile phase for all the pesticides studied except propiconazole, for which only the 2 diastereomers were baseline separated. This method allows determination of the enantiomers or stereoisomers of the above pesticides in soil. The strategy was as follows: (1) First, the total concentration(s) of the enantiomer pair(s) of a chiral pesticide in soil was (were) determined by a newly developed matrix solid-phase dispersion (MSPD) procedure, followed by silica-based LC quantification. The recoveries ranged from 76.5 to 93.6% with relative standard deviations of 6.0%. (2) Second, the enantiomeric ratio(s) (ER(s)) of the chiral pesticide was (were) determined by LC with a chiral stationary phase after fractionation of the MSPD extract by silica-based LC. The determined ERs or stereoisomeric ratio(s) (SR(s); for propiconazole, only the SR of the 2 diastereomers was determined) in soil samples spiked with the above 8 racemic pesticides agreed with those of the corresponding standard solutions. (3) Third, based on the total concentrations and the corresponding ERs, the concentration of each enantiomer in soil was calculated. The proposed method is rapid, precise, and sensitive, and is appropriate for the investigation of the stereo- and enantioselective degradation of pesticides in environmental media. 相似文献
992.
Chen T Wang L Jiang G Wang J Dong X Wang X Zhou J Wang C Wang W 《The journal of physical chemistry. B》2005,109(10):4624-4630
The electrochemical behavior of poly(ferrocenyldimethylsilane-b-dimethylsiloxane) (PFDMS-b-PDMS) films deposited on a glassy carbon electrode was investigated by means of cyclic voltammetry (CV). The influences of the solvent, film thickness, temperature, and PDMS block length in PFDMS-b-PDMS on the electrode process were discussed. It was found that in 0.1 M aqueous LiClO(4) the electrochemical processes of the films on a glassy carbon electrode were complex and have a low rate of electron transport and mass diffusion. The kinetic parameters obtained indicated that the electrode process was controlled by both the electrode reaction and mass diffusion. 相似文献
993.
994.
Conformations of fold part in isotactic polypropylene lamella with diamond lattice model 总被引:1,自引:0,他引:1
Inthefieldofpolymerphysics,thecrystallinestateofpolymershaslongbeenofinterest.TheexistenceofpolymersinglecrystalwasfirstdiscoveredbyJaccodine[1]in1955.Thethicknessofmostsolutiongrowthcrystalsisfoundtobeoftheorderof~10nm.Thisobservationissomewhatsurprising.S… 相似文献
995.
Kebin Zhou Hongde Chen Qun Tian Dixin Shen Xiaobai Xu 《Reaction Kinetics and Catalysis Letters》2002,77(1):65-72
Pd/LaFe0.8Co0.2O3 was found to be more active than the Pd/Al2O3 catalyst in propane oxidation. Activity and the oxidation state of palladium were strongly affected by the redox ratio (S) of reactants. Higher propane conversion could be obtained under rich conditions at higher temperatures, contributing to steam reforming. The presence of excess steam would bring about inhibition and oscillation. 相似文献
996.
Jun Zhang Kaimin Liu Gengmei Xing Tongxiang Ren Shukuan Wang 《Journal of Radioanalytical and Nuclear Chemistry》2007,272(3):605-609
Gd@C82(OH)40 has been developed as a new generation of MRI contrast agent. But recently, it was found that Gd@C82(OH)
x
with a larger number of OH (x>36) would lead to cage break and hence, release of highly toxic Gd ions. We synthesized the more stable Gd@C82(OH)
x
with less OH-number, Gd@C82(OH)16, and studied its proton relaxivity and MRI images. The results indicate that Gd@C82(OH)16 also gives high proton relaxivity, even higher than that of (NMG)2-Gd-DTPA. The bio-distribution indicated that Gd@C82(OH)16 tends to be entrapped in the liver and kidney and remained in tissue for about 2 hours. The results suggest that the more
stable metallofullerene derivative Gd@C82(OH)16 can be the potential candidate of the new MRI contrast agent. 相似文献
997.
998.
表面包覆改性对纳米CeO2分散性的影响 总被引:1,自引:0,他引:1
了改善纳米CeO2在Zn-Al类共晶合金中的分散性,采用超声液相包覆法对纳米CeO2进行表面活性剂表面包覆改性,并用AES测定了包覆层的厚度,用TEM研究了CeO2的团聚状况,用TGA分析了有机物包覆层的炭化温度范围,最后用FE-SEM观察了CeO2在复合材料中的分散情况。结果表明,包覆在纳米CeO2表面的厚度约为20 nm的表面活性剂层提高了微粒的分散性,而且该包覆层在495℃时已经炭化。热力学计算的结果也表明,炭化层能与氧化膜反应,该反应提高了CeO2与基体间的润湿性,并使其均匀分布在基体合金中。 相似文献
999.
Macroporous poly(methyl methacrylate) produced by phase separation during polymerisation in solution
A. Serrano Aroca M. Monleón Pradas J. L. Gómez Ribelles 《Colloid and polymer science》2007,285(7):753-760
Polymethyl methacrylate (PMMA) sponges were obtained by polymerization in a solution with monomer/ethanol ratios up to 20:80.
The material obtained after the elimination of the solvent present a homogeneous distribution of dispersed pores up to a monomer/ethanol
ratio lower than 40:60. For higher ethanol contents in the reacting mixture, the morphology of the sponge corresponds to a
network of PMMA microparticles, leaving large empty spaces leading to highly porous structure. The monomer/ethanol ratio during
polymerization has a large influence on the porosity, thermal, and mechanical properties of the material and, for large solvent
contents, on the size of the polymer microparticles. 相似文献
1000.
The carbobenzyloxy (cbz) protecting group is evaluated for it's potential to enhance the resolution of chiral amine enantiomers using high-performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC). A series of cbz derivatives of commercially available racemates was prepared and analyzed by enantioselective chromatography using a variety of mobile phases and polysaccharide and Pirkle-type chiral stationary phases (CSPs). The cbz-derivatized product consistently demonstrated enhanced chiral resolution under HPLC and SFC conditions. Improved selectivity and resolution combined with an automated preparative HPLC or SFC system can lead to the rapid generation of highly purified enantiomers of desirable starting materials, intermediates or final products. 相似文献