全文获取类型
收费全文 | 7559篇 |
免费 | 865篇 |
国内免费 | 599篇 |
专业分类
化学 | 5495篇 |
晶体学 | 66篇 |
力学 | 338篇 |
综合类 | 35篇 |
数学 | 831篇 |
物理学 | 2258篇 |
出版年
2023年 | 113篇 |
2022年 | 122篇 |
2021年 | 163篇 |
2020年 | 209篇 |
2019年 | 153篇 |
2018年 | 161篇 |
2017年 | 147篇 |
2016年 | 243篇 |
2015年 | 241篇 |
2014年 | 317篇 |
2013年 | 373篇 |
2012年 | 479篇 |
2011年 | 528篇 |
2010年 | 385篇 |
2009年 | 354篇 |
2008年 | 368篇 |
2007年 | 366篇 |
2006年 | 357篇 |
2005年 | 314篇 |
2004年 | 250篇 |
2003年 | 176篇 |
2002年 | 201篇 |
2001年 | 148篇 |
2000年 | 151篇 |
1999年 | 147篇 |
1998年 | 126篇 |
1997年 | 125篇 |
1996年 | 128篇 |
1995年 | 84篇 |
1994年 | 120篇 |
1993年 | 97篇 |
1992年 | 108篇 |
1991年 | 90篇 |
1990年 | 63篇 |
1989年 | 82篇 |
1988年 | 80篇 |
1987年 | 99篇 |
1986年 | 52篇 |
1985年 | 62篇 |
1984年 | 61篇 |
1983年 | 47篇 |
1982年 | 53篇 |
1981年 | 51篇 |
1980年 | 42篇 |
1979年 | 46篇 |
1978年 | 46篇 |
1977年 | 55篇 |
1976年 | 58篇 |
1975年 | 36篇 |
1974年 | 43篇 |
排序方式: 共有9023条查询结果,搜索用时 125 毫秒
81.
甲醇与乙醇一步合成异丁醛用CuO-ZnO/Al2O3催化剂 总被引:20,自引:0,他引:20
在CuO-ZnO/Al2O3催化剂上,甲醇与乙醇在常压和210 ̄360℃温度下一步反应合成异丁醛。利用正交设计方法,研究了CuO-ZnO/Al2O3体系催化剂组成和制备条件等因素对催化剂催化性能的影响。结果表明,催化剂中CuO含量对催化剂的催化活性影响最大,而ZnO含量则主要影响催化剂对异丁醛的选择性。催化剂制备条件,如沉淀温度、陈化时间等,也不同程度也影响催化剂的催化性能。XRD和BET表征结果 相似文献
82.
Meerwein reactions of 1,5-anthraquinone bis(diazonium hydrogen sulfate) with sufficiently activated olefins, such as acrylonitrile, acrylic esters, methacrylonitrile and its esters, styrene and α-methylstyrene yielded derivatives of the new heterocyclic system anthra[9,1-bc:10,5-b'c']-2,3,7,8-tetrahydrodipyran. Isolation of derivatives was realized and yields enhanced by using dimethyl methylphosphonate as reaction medium. Compounds 5–18 were isolated as mixtures of diastereomers, some of which were separated by crystallization. Proof of structure and stereochemistry was obtained for some of the compounds by X-ray crystallographic analysis. The 1H- and 13C-nmr data provided further support. Surprisingly large chemical shift differences between some of the proton signals of isomers were observed although the two asymmetric centers are far apart through bonds and through space. The mechanism of formation is explained on the basis of redox modulation of a charge transfer complex in which the aliphatic radical attacks the oxygen atom of the anthraquinone. It is proposed to classify this new type of reaction as an intramolecular Meerwein reaction. 相似文献
83.
本工作用四种较新的质谱技术(碰撞诱导解离(CID),电荷剥离(CS),电荷分离(CSe),电子捕获诱导解离(ECID)谱研究了邻、间、对溴甲苯和溴化苄四种异构体。四种技术在一定程度上都可区分这些异构体。其中比较四种异构体ECID谱中ECID峰的相对强度可得到满意的区分效果。 相似文献
84.
85.
86.
ADSORPTION OF 2,4-DICHLOROBENZOXYACETIC ACID ONTO HYPERCROSSLINKED RESIN MODIFIED BY PHENOLIC HYDROXYL GROUP (AM-1) 总被引:2,自引:0,他引:2
INTRODUCTIONAdsorption has been considered to be the best available technology for removing organics from water in the USSafe Drinking Water Act[1]. Due to many drawbacks of activated carbon[2-7], the most widely used adsorbent,hypercrosslinked polymeric adsorbents developed by Tsyurupa and Davankov[8] have been increasingly viewedas an alternative to activated carbon for selective removal of specific organic substances from contaminatedwater[9-12], and series of researches have been do… 相似文献
87.
The formation of covalently linked composites of multi–walled carbon nanotubes (MWCNT) and glucose oxidase (GOD) with high-function
density for use as a biosensing interface is described. The reaction intermediates and the final product were characterized
by using FT–IR spectroscopy, and the MWCNT-coated GOD nanocomposites were examined by atomic force microscopy (AFM) and transmission
electron microscopy (TEM). Interestingly, it was found that the GOD–MWCNT composites are highly water soluble. Electrochemical
characterization of the GOD–MWCNT composites that were modified on a glassy carbon electrode shows that the covalently linked
GOD retains its bioactivity and can specifically catalyze the oxidation of glucose. The oxidation current shows a linear dependence
on the glucose concentration in the solution in the range of 0.5–40 mM with a detection limit of 30 μM and a detection sensitivity
of 11.3 μA/mMcm2. The present method may provide a way to synthesize MWCNT related composites with other biomolecules and for the construction
of enzymatic reaction-based biofuel cells and biosensors.
Supported by grants from the National Natural Science Foundation of China (NSFC, No. 20125515; 90206037; 20375016) and the
Natural Science Foundation of Jiangsu Province (Grant No. BK 2004210) 相似文献
88.
89.
JunZhou Yong-zhiQiu Xiao-pengZang Chang-wangPan QiangChen JianShen Si-congLin 《高分子科学》2005,(1):93-102
N,N-dimethyl-N-methacryloyloxyethyl-N-carboxyethyl ammonium (DMMCA) was graft-copolymerized onto the surface of segmented poly(ether urethane) (SPEU) and PE film. The carboxybetaine structure on SPEU and PE film surfaces was confirmed by ATR-FTIR, XPS and water contact angle measurements. Through the experiments with platelet adhesion and protein adhesion assay in vitro, the two materials studied, including poly-DMMCA gel, all show excellent nonthrombogenicity. This confirms once again that the zwitterionic molecular structure on the surfaces of materials is essential for improving their nonthrombogenicity and biocompatibility. 相似文献
90.