全文获取类型
收费全文 | 7906篇 |
免费 | 963篇 |
国内免费 | 689篇 |
专业分类
化学 | 3626篇 |
晶体学 | 60篇 |
力学 | 310篇 |
综合类 | 35篇 |
数学 | 583篇 |
物理学 | 2052篇 |
综合类 | 2892篇 |
出版年
2024年 | 30篇 |
2023年 | 137篇 |
2022年 | 172篇 |
2021年 | 210篇 |
2020年 | 225篇 |
2019年 | 165篇 |
2018年 | 179篇 |
2017年 | 158篇 |
2016年 | 255篇 |
2015年 | 289篇 |
2014年 | 389篇 |
2013年 | 416篇 |
2012年 | 522篇 |
2011年 | 553篇 |
2010年 | 427篇 |
2009年 | 406篇 |
2008年 | 421篇 |
2007年 | 446篇 |
2006年 | 377篇 |
2005年 | 334篇 |
2004年 | 287篇 |
2003年 | 226篇 |
2002年 | 265篇 |
2001年 | 243篇 |
2000年 | 197篇 |
1999年 | 284篇 |
1998年 | 253篇 |
1997年 | 233篇 |
1996年 | 220篇 |
1995年 | 176篇 |
1994年 | 179篇 |
1993年 | 155篇 |
1992年 | 126篇 |
1991年 | 115篇 |
1990年 | 108篇 |
1989年 | 115篇 |
1988年 | 77篇 |
1987年 | 86篇 |
1986年 | 39篇 |
1985年 | 26篇 |
1984年 | 10篇 |
1983年 | 9篇 |
1982年 | 5篇 |
1981年 | 8篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有9558条查询结果,搜索用时 15 毫秒
101.
用丘氏气提定氮器测定植物中的氮 总被引:1,自引:0,他引:1
研究了用气提滴定法测定植物中氮的条件和方法.实验结果表明,与蒸馏法相比,方法简便安全,结果准确 相似文献
102.
103.
104.
In this review, we summarize the number of scientific publications in the field of FP/FA sensor in recent five years, and introduce the recent progress of FP/FA sensor based on nanomaterial. The various analytical applications of FP/FA sensor based on nanomaterial are discussed. We also provide perspectives on the current challenges and future prospects in the promising field. 相似文献
105.
Renqing Lü 《天然气化学杂志》2005,14(2):119-124
The density functional theory and the cluster model methods have been employed to investigate the interactions between ethanethiol and HZSM-5 zeolites. Molecular complexes formed by the adsorption of ethanethiol on silanol H3SiOH with two coordination forms, model Bronsted acid sites of zeolite cluster H3Si(OH)Al(OH)2SiH3 interaction with ethanethiol, aluminum species adsorbed ethanethiol have been comparatively studied. Full optimization and frequency analysis of all cluster models have been carried out using B3LYP hybrid method at 3-21G basis level for hydrogen atoms and 6-31G(d) basis set level for silicon, aluminum, oxygen, carbon, and sulfur atoms. The structures and energy changes of different coordination forms of H3Si(OH)Al(OH)2SiH3-ethanethiol, silanol-ethanethiol and Al(OH)3-ethanethiol have been studied. The calculated results showed the nature of interactions was van der Waals force as exhibited by not much change in geometric structures and properties. The preference order of ethanethiol adsorbed on HZSM-5 zeolite may be residual aluminum species, bridging hydroxyl groups and silanol OH groups from the adsorption heat. The adsorbed models of protonized ethanethiol on bridging hydroxyl OH groups and linear hydrogen bonded ethanethiol on bridging OH groups suggested in literature might not exist as revealed by this theoretical calculation. Possible adsorption models were obtained for the first time. 相似文献
106.
107.
108.
Liu L Qiu S Wang B Zhang W Lu P Xie Z Hanif M Ma Y Shen J 《The journal of physical chemistry. B》2005,109(49):23366-23370
We have investigated the thermal degradation in air by Fourier transform infrared spectroscopy of a ladder-type copolymer containing fluorene units in the backbone (Me-LPF), to reveal the formation of the ketonic defects. As thermal treatment of Me-LPF film at 200 degrees C in air proceeds, a new group of complex absorption bands due to degradation products arises in the range between 1800 and 1600 cm(-1). The observed overlapping bands were separated and assigned by utilizing the second-derivative IR spectral analysis, which can narrow the peak width to one-third of the originals and thereby eases the analysis. The degraded products were assigned as fluorenone (1718 cm(-1)) and benzophenone (Ar-(C=O)-Ar) (1665 cm(-1)), formed by the oxidation of the backbone, and acylphenone (Ar-(C=O)-R) (1685 cm(-1)) from the side chain. The fluorenone was found to be the major component among the degraded products in the main chain, and the time and temperature dependence indicated that the oxidation is a kind of autocatalytic radical-chain process. The oxidation can reach a very high degree (approximately 30% for 6 h oxidation at 240 degrees C estimated by absorption of the alkyl). Our results suggest the possibility of the oxidation of the 9-bialkylfluorene sites. We propose that the degradation of the alkyl in the side chain can help the radicals to propagate in the chain reaction. 相似文献
109.
The additive Pummerer reaction of several heteroaromatic sulfilimines has been investigated. The overall process involves the reaction of the sulfilimine with TFAA to produce a transient N-tosyl-N-trifluoroacetyl sulfonium ion. Nucleophilic attack at the adjacent vinyl carbon results in the ejection of the sulfonamide group and the resulting thionium ion loses a proton to give the observed product. 相似文献
110.