全文获取类型
收费全文 | 36996篇 |
免费 | 5716篇 |
国内免费 | 3645篇 |
专业分类
化学 | 25716篇 |
晶体学 | 358篇 |
力学 | 2227篇 |
综合类 | 241篇 |
数学 | 4280篇 |
物理学 | 13535篇 |
出版年
2024年 | 111篇 |
2023年 | 759篇 |
2022年 | 1224篇 |
2021年 | 1340篇 |
2020年 | 1443篇 |
2019年 | 1329篇 |
2018年 | 1150篇 |
2017年 | 1067篇 |
2016年 | 1610篇 |
2015年 | 1633篇 |
2014年 | 2004篇 |
2013年 | 2578篇 |
2012年 | 3253篇 |
2011年 | 3210篇 |
2010年 | 2180篇 |
2009年 | 2050篇 |
2008年 | 2203篇 |
2007年 | 1960篇 |
2006年 | 1842篇 |
2005年 | 1539篇 |
2004年 | 1301篇 |
2003年 | 984篇 |
2002年 | 894篇 |
2001年 | 738篇 |
2000年 | 691篇 |
1999年 | 829篇 |
1998年 | 693篇 |
1997年 | 635篇 |
1996年 | 705篇 |
1995年 | 601篇 |
1994年 | 550篇 |
1993年 | 470篇 |
1992年 | 449篇 |
1991年 | 364篇 |
1990年 | 324篇 |
1989年 | 236篇 |
1988年 | 216篇 |
1987年 | 198篇 |
1986年 | 141篇 |
1985年 | 156篇 |
1984年 | 135篇 |
1983年 | 116篇 |
1982年 | 80篇 |
1981年 | 59篇 |
1980年 | 49篇 |
1979年 | 33篇 |
1978年 | 26篇 |
1976年 | 27篇 |
1975年 | 31篇 |
1974年 | 23篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
871.
甲烷氧化偶联反应在不同催化剂体系上动力学的研究各不相同,我们已报道了La-Ba-O系复合氧化物催化剂对此反应的优良催化性能。本文则是对该催化剂上的甲烷氧化偶联反应的动力学进行初步探讨。 相似文献
872.
利用X-射线衍射分析和电子显微镜分析的结果,确定堇青石窑具的多晶结构和各晶相的含量;结合窑具性能的研究,确定堇青石窑具具有优良性能的适宜配方。 相似文献
873.
874.
875.
Nonisothermal kinetics of the solid‐solid phase transition in (n‐C10H21NH3)2ZnCl4(C10Zn), (n‐C16H33NH3)2ZnCl4(C16Zn) and their binary system were determined by Kissinger and Ozawa methods from DSC measurements. The activation energy Ea of the binary system shows a waving dependence on WC10Zn%, which is caused by not only an intermediate (C10H21NH3)(n‐C16H33NH3)ZnCl4 but also three solid solution ranges (α, β, γ) in the phase diagram of C10Zn‐C16Zn. The variations of the layer d‐spacing are also convenient for the above result. 相似文献
876.
Chemical Constituents of Ailanthus triphysa 总被引:2,自引:0,他引:2
Two new compounds,8(14),15-isopimaradiene-2α,3α,19-triol(1),and 6α,7β-dihydroxy-17(20)-cis-5α-pregna-16-one(2),together with four known copounds,a oxygenated rare phyllocladane,phyllocladan-16α,19-diol(3),kaempferol-3-0-β-D-galactopyranosied,kaempferol-3-0-α-L-rhamnopyranoside and scopoletin, were isolated from the leaves of Ailanthus tripysa.Structures of 1-3 were elucidated on the basis of spectroscopic data as compared with related compounds. 相似文献
877.
In Situ IR Spectroscopic Studies on Molybdenum Nitride Catalysts: Active Sites and Surface Reactions
Recent IR spectroscopic studies on the surface properties of fresh Mo2N/-Al2O3 catalyst are presented in this paper. The surface sites of fresh Mo2N/-Al2O3, both Mo+ (0<<2) and N sites, are probed by CO adsorption. Two characteristic IR bands were observed at 2045 and 2200 cm-1, due to linearly adsorbed CO on Mo and N sites, respectively. The surface N sites are highly reactive and can react with adsorbed CO to form NCO species. Unlike adsorbed CO on reduced passivated one, the adsorbed CO on fresh Mo2N/-Al2O3 behaves similarly to that of group VIII metals, suggesting that fresh nitride resembles noble metals. It is found that the surface of Mo nitrides slowly transformed into sulfide under hydrotreating conditions, which could be the main reason for the activity drop of molybdenum nitride catalysts in the presence of sulfur-containing species. Some surface reactions, such as selective hydrogenation of 1,3-butadiene, isomerization of 1-butene, and hydrodesulfurization of thiophene, were studied on both fresh and reduced passivated Mo2N/-Al2O3 catalysts using IR spectroscopy. The mechanisms of these reactions are proposed. The adsorption and reaction behaviors of these molecules on fresh molybdenum nitride also resemble those on noble metals, manifesting the unique properties of fresh molybdenum nitride catalysts. Mo and N sites are found to play different roles in the adsorption and catalytic reactions on the fresh Mo2N/-Al2O3 catalyst. Generally, Mo sites are the main active sites for the adsorption and reactions of adsorbates; N sites are not directly involved in catalytic reactions but they modify the electronic properties of Mo sites. 相似文献
878.
Ren Xiaoming Wu Peiheng Zhang Wenwei Meng Qingjin Chen Xiaoyuan 《Transition Metal Chemistry》2002,27(4):394-397
Ion-pair complexes consisting of the [Fe(mnt)2]– anion and [RBzPy]+ cations [R = NO2
(1) or Cl (2)] have been prepared and characterized by elemental analysis and i.r. spectroscopy. The single crystal structure of (1) shows that the [Fe(mnt)2]– anions are dimeric, and that the coordination geometry of each Fe atom is square pyramidal. The most significant structural feature of (1) is that anions and cations form completely separate columns and – interactions occur within the cation columns. Thermal variations of
m
T for the complexes indicated antiferromagnetic coupling within the dimer. 相似文献
879.
A novel heteronuclear complex, [Tb2Zn2L10(bipy)(H2O)2]2 (HL = -methylacrylic acid, bipy = 4,4-bipyridine), was synthesized and characterized by elemental analysis and i.r. absorption spectra. Its structure, determined by X-ray diffraction, is a discrete octanuclear molecule, in which two ZnII ions are linked by a bipy molecule and between TbIII ions, or TbIII and ZnII ions are bridged by bidentate -methylacrylato groups. The temperature dependence of the magnetic susceptibility of the complex shows strong ferromagnetic interaction between the terbium atoms. 相似文献
880.
Zartler ER Hanson J Jones BE Kline AD Martin G Mo H Shapiro MJ Wang R Wu H Yan J 《Journal of the American Chemical Society》2003,125(36):10941-10946
The crucial step in drug discovery is the identification of a lead compound from a vast chemical library by any number of screening techniques. NMR-based screening has the advantage of directly detecting binding of a compound to the target. The spectra resulting from these screens can also be very complex and difficult to analyze, making this an inefficient process. We present here a method, RAMPED-UP NMR, (Rapid Analysis and Multiplexing of Experimentally Discriminated Uniquely Labeled Proteins using NMR) which generates simple spectra which are easy to interpret and allows several proteins to be screened simultaneously. In this method, the proteins to be screened are uniquely labeled with one amino acid type. There are several benefits derived from this unique labeling strategy: the spectra are greatly simplified, resonances that are most likely to be affected by binding are the only ones observed, and peaks that yield little or no information upon binding are eliminated, allowing the analysis of multiple proteins easily and simultaneously. We demonstrate the ability of three different proteins to be analyzed simultaneously for binding to two different ligands. This method will have significant impact in the use of NMR spectroscopy for both the lead generation and lead optimization phases of drug discovery by its ability to increase screening throughput and the ability to examine selectivity. To the best of our knowledge, this is the first time in any format that multiple proteins can be screened in one tube. 相似文献