全文获取类型
收费全文 | 49788篇 |
免费 | 8910篇 |
国内免费 | 7550篇 |
专业分类
化学 | 36711篇 |
晶体学 | 774篇 |
力学 | 3253篇 |
综合类 | 557篇 |
数学 | 5794篇 |
物理学 | 19159篇 |
出版年
2024年 | 109篇 |
2023年 | 841篇 |
2022年 | 1376篇 |
2021年 | 1678篇 |
2020年 | 1962篇 |
2019年 | 1984篇 |
2018年 | 1703篇 |
2017年 | 1740篇 |
2016年 | 2302篇 |
2015年 | 2487篇 |
2014年 | 3044篇 |
2013年 | 3938篇 |
2012年 | 4506篇 |
2011年 | 4734篇 |
2010年 | 3478篇 |
2009年 | 3592篇 |
2008年 | 3806篇 |
2007年 | 3279篇 |
2006年 | 3043篇 |
2005年 | 2537篇 |
2004年 | 1904篇 |
2003年 | 1569篇 |
2002年 | 1551篇 |
2001年 | 1290篇 |
2000年 | 1067篇 |
1999年 | 961篇 |
1998年 | 782篇 |
1997年 | 666篇 |
1996年 | 614篇 |
1995年 | 521篇 |
1994年 | 534篇 |
1993年 | 398篇 |
1992年 | 336篇 |
1991年 | 276篇 |
1990年 | 286篇 |
1989年 | 178篇 |
1988年 | 167篇 |
1987年 | 154篇 |
1986年 | 152篇 |
1985年 | 124篇 |
1984年 | 75篇 |
1983年 | 63篇 |
1982年 | 52篇 |
1981年 | 42篇 |
1980年 | 28篇 |
1979年 | 38篇 |
1978年 | 25篇 |
1977年 | 24篇 |
1975年 | 22篇 |
1972年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Shen R Lin CT Bowman EJ Bowman BJ Porco JA 《Journal of the American Chemical Society》2003,125(26):7889-7901
The total synthesis and stereochemical assignment of the potent antitumor macrolide lobatamide C, as well as synthesis of simplified lobatamide analogues, is reported. Cu(I)-mediated enamide formation methodology has been developed to prepare the highly unsaturated enamide side chain of the natural product and analogues. A key fragment coupling employs base-mediated esterification of a beta-hydroxy acid and a salicylate cyanomethyl ester. Three additional stereoisomers of lobatamide C have been prepared using related synthetic routes. The stereochemistry at C8, C11, and C15 of lobatamide C was assigned by comparison of stereoisomers and X-ray analysis of a crystalline derivative. Synthetic lobatamide C, stereoisomers, and simplified analogues have been evaluated for inhibition of bovine chromaffin granule membrane V-ATPase. The salicylate phenol, enamide NH, and ortho-substitution of the salicylate ester have been shown to be important for V-ATPase inhibitory activity. 相似文献
962.
963.
Wen Wei Lai Teh Chou Chang 《Journal of polymer science. Part A, Polymer chemistry》1995,33(7):1075-1083
A series of crosslinkable thermotropic liquid crystalline poly(ether-ester)s and copoly(ether-ester)s was prepared. All of the polymers were crosslinked by thermal treatment or photo-irradiation upon heating. The thermal stability and thermal crosslinking reaction of these polymers were investigated. These polymers also could be crosslinked by copolymerization with vinyl monomers, such as styrene or methyl methacrylate. The crosslinked polymers exhibited thermotropic liquid crystalline behavior after softening by heating. The phase behavior of linear polymers and crosslinked polymers was studied by differential scanning calorimetry (DSC) and an optical polarizing microscope equipped with a heating stage. © 1995 John Wiley & Sons, Inc. 相似文献
964.
965.
The convection in atmosphere discussed in ref. [1] is rigorously treated by considering the variation of environmental temperature
with the height. This represents an example of applications of the elementary catastrophe theory in Hamiltonian systems. 相似文献
966.
A one-step pathway has been adopted to fabricate biodegradable polylactide (PLA) hollow spheres by interfacial polymerization
in an oil-in-water emulsion system. The mechanism of sphere formation is suggested with respect to interfacial cross-linking
polymerization and subsequent precipitation of the unreacted PLA macromonomers onto the preformed shells. Their hollow nature
and morphology were verified by transmission electron microscopy and scanning electron microscope characterizations. 相似文献
967.
β沸石分子筛固体酸的制备及其在异丁烷和丁烯烷基化反应中的应用 总被引:1,自引:0,他引:1
以四乙基溴化铵(TEABr)为模板剂,硅胶为硅源,铝酸钠为铝源,采用水热晶化法合成β沸石.研究了其晶化温度、晶化时间、硅铝比、碱度、水含量等各种因素对晶化过程和产物性质的影响.得出适宜的初始凝胶组成为SiO2和Al2O3的比值在25-60之间,TEABr和SiO2的比值在0.32~0.4之间,OH^-和SiO2的比值在0.2~0.23之间,H2O和SiO2的比值在8.6~18.0之间.通过XRD,ICP,IR,SEM和NH3-TPD方法分别对样品的结构和性质进行了表征.烷基化反应的结果表明,β分子筛具有较为适宜的孔结构和反应活性. 相似文献
968.
Bernd GüNTHER 《中国科学B辑(英文版)》2007,50(6)
The interaction between stabilizers and nanoparticles is one of the important factors to prepare stable magnetic fluids. The magnetic nano-size Fe3O4 core with single domain and the average grain size around 8-12 nm were prepared by chemical precipitation method. The O/Fe molar ratio of the particle surface was measured by X-ray photoelectron spectroscopy (XPS). The heat effects of stabilizers ad- sorption on nanoparticles were measured by solution calorimetry. The excess amount of oxygen was possibly the result of the hydroxygen formed on the surface of the nanoparticles. The heat effects showed that compounds containing carboxyl groups can be adsorbed chemically on magnetite by forming chemical bonds. The other stabilizers involving NH-groups, such as polyethylene-imine, can be adsorbed physically. The exothermic value is about half of the former case. 相似文献
969.
970.
秩消因子分析—荧光光度测定某些食物中的营养元素硒 总被引:1,自引:0,他引:1
本文应用秩消因子分析-荧光光度法测定了食物中营养元素硒的含量,与文献〔1〕报道的DAN-荧光光度法测定结果比较,具有良好的统计学一致性,从而为简化实验操作,改善痕量硒测定的准确性和灵敏度提供了新的可能。 相似文献