全文获取类型
收费全文 | 2846篇 |
免费 | 60篇 |
国内免费 | 51篇 |
专业分类
化学 | 2008篇 |
晶体学 | 13篇 |
力学 | 56篇 |
数学 | 511篇 |
物理学 | 369篇 |
出版年
2021年 | 21篇 |
2017年 | 17篇 |
2016年 | 50篇 |
2015年 | 29篇 |
2014年 | 57篇 |
2013年 | 109篇 |
2012年 | 97篇 |
2011年 | 135篇 |
2010年 | 58篇 |
2009年 | 46篇 |
2008年 | 108篇 |
2007年 | 119篇 |
2006年 | 81篇 |
2005年 | 104篇 |
2004年 | 87篇 |
2003年 | 68篇 |
2002年 | 56篇 |
2001年 | 20篇 |
2000年 | 25篇 |
1999年 | 22篇 |
1998年 | 23篇 |
1997年 | 31篇 |
1996年 | 30篇 |
1995年 | 29篇 |
1994年 | 28篇 |
1992年 | 25篇 |
1991年 | 32篇 |
1990年 | 21篇 |
1989年 | 19篇 |
1988年 | 38篇 |
1987年 | 48篇 |
1986年 | 37篇 |
1985年 | 46篇 |
1984年 | 49篇 |
1983年 | 35篇 |
1982年 | 39篇 |
1981年 | 44篇 |
1980年 | 39篇 |
1979年 | 39篇 |
1978年 | 68篇 |
1977年 | 50篇 |
1976年 | 53篇 |
1975年 | 24篇 |
1974年 | 40篇 |
1973年 | 38篇 |
1972年 | 24篇 |
1971年 | 16篇 |
1969年 | 16篇 |
1885年 | 23篇 |
1884年 | 26篇 |
排序方式: 共有2957条查询结果,搜索用时 31 毫秒
71.
The three main types of hydroxyl groups on a silica surface are classified as isolated, hydrogen bonded, and inaccessible. The isolated and hydrogen bonded groups are the most important as these readily exchange with D(2)O and thus are exposed to reactant molecules. However, it has generally been accepted that the inaccessible groups do not participate in surface reactions as only a small fraction of these groups exchange with D(2)O. It is shown that the inaccessible hydroxyl groups on nonporous fumed silica and mesoporous MCM-48 silica powders and films fully exchange with D(2)O and are reactive with octadecylydimethylchlorosilane when supercritical CO(2) is used as the solvent. Furthermore, it is found that the CO(2) penetrating the regions containing the inaccessible groups is not removed by simple evacuation but rather slowly diffuses from the silica over periods of months. 相似文献
72.
Richard A. Bartsch David A. Babb Bronislaw P. Czech Dhimant H. Desai Eddy Chapoteau Carl R. Gebauer Wolodymyr Zazulak Anad Kumar 《Journal of inclusion phenomena and macrocyclic chemistry》1990,9(2):113-123
Four novel chromogenic benzocryptands and one side-armed chromogenic cryptand have been synthesized and their complexing abilities for alkali metal cations are described. 相似文献
73.
74.
75.
Ruthenium(II) complexes of monodonor ligands: efficient reagents for asymmetric ketone hydrogenation
Xu YA Clarkson GC Docherty G North CL Woodward G Wills M 《The Journal of organic chemistry》2005,70(20):8079-8087
[Chemical reaction: See text] A series of BINOL-derived ligands have been prepared and incorporated into ruthenium(II) complexes containing a diamine ligand. The complexes have proven to be excellent catalysts for the asymmetric hydrogenation of ketones, giving reduction products with enantiomeric excesses of up to 99%. 相似文献
76.
77.
An investigation to determine the compounds present in the Ln2O3Nb2O5 systems (Ln = La, Gd, and Y) and attempts to activate such compounds with Ti, In, Sb, Bi, Eu, and Tb revealed that only the orthoniobate (LnNbO4) compound was a suitable host lattice and LnNbO4:Bi under 2537 Å excitation produced the best phosphors. GdNbO4:Bi is a brighter phosphor than the La and Y analogs, emitting at slightly higher energies. The position of the GdNbO4:Bi peak emission at 4500 Å is independent of the activator concentration whereas peak emissions for LaNbO4:Bi and YNbO4:Bi move to lower energies with increasing Bi concentration. 相似文献
78.
79.
Transient streaming potential in a finite length microchannel 总被引:4,自引:0,他引:4
Mansouri A Scheuerman C Bhattacharjee S Kwok DY Kostiuk LW 《Journal of colloid and interface science》2005,292(2):567-580
Pressure-driven flow of an electrolyte solution in a microchannel with charged solid surfaces induces a streaming potential across the microchannel. Such a flow also causes rejection of ions by the microchannel, leading to different concentrations in the feed and permeate reservoirs connecting the capillary, which forms the basis of membrane based separation of electrolytes. Modeling approaches traditionally employed to assess the streaming potential development and ion rejection by capillaries often present a confusing picture of the governing electrochemical transport processes. In this paper, a transient numerical simulation of electrochemical transport process leading to the development of a streaming potential across a finite length circular cylindrical microchannel connecting two infinite reservoirs is presented. The solution based on finite element analysis shows the transient development of ionic concentrations, electric fields, and the streaming potential over the length of the microchannel. The transient analysis presented here resolves several contradictions between the two types of modeling approaches employed in assessing streaming potential development and ion rejection. The simulation results show that the streaming potential across the channel is predominantly set up at the timescale of the developing convective transport, while the equilibrium ion concentrations are developed over a considerably longer duration. 相似文献
80.
Two Novel long chain fatty acids, (5Z, 9Z)-6-bromo-25-methyl-5,9-hexacosadienoic and (5Z, 9Z)-6-bromo-24-methyl-5,9-hexacosadienoic acids were found in the phospholipids of and . Their structures were elucidated with the help of CI-EI7MS and a homogeneous hydrogenation catalyst. 相似文献