全文获取类型
收费全文 | 28592篇 |
免费 | 4687篇 |
国内免费 | 3327篇 |
专业分类
化学 | 20377篇 |
晶体学 | 362篇 |
力学 | 1821篇 |
综合类 | 305篇 |
数学 | 3186篇 |
物理学 | 10555篇 |
出版年
2024年 | 48篇 |
2023年 | 588篇 |
2022年 | 851篇 |
2021年 | 973篇 |
2020年 | 1223篇 |
2019年 | 1104篇 |
2018年 | 915篇 |
2017年 | 903篇 |
2016年 | 1309篇 |
2015年 | 1245篇 |
2014年 | 1562篇 |
2013年 | 1996篇 |
2012年 | 2503篇 |
2011年 | 2554篇 |
2010年 | 1758篇 |
2009年 | 1699篇 |
2008年 | 1834篇 |
2007年 | 1646篇 |
2006年 | 1566篇 |
2005年 | 1294篇 |
2004年 | 1058篇 |
2003年 | 880篇 |
2002年 | 782篇 |
2001年 | 687篇 |
2000年 | 614篇 |
1999年 | 699篇 |
1998年 | 571篇 |
1997年 | 488篇 |
1996年 | 511篇 |
1995年 | 430篇 |
1994年 | 415篇 |
1993年 | 359篇 |
1992年 | 290篇 |
1991年 | 252篇 |
1990年 | 252篇 |
1989年 | 169篇 |
1988年 | 134篇 |
1987年 | 91篇 |
1986年 | 110篇 |
1985年 | 72篇 |
1984年 | 39篇 |
1983年 | 43篇 |
1982年 | 45篇 |
1981年 | 23篇 |
1980年 | 11篇 |
1979年 | 4篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
991.
992.
993.
Xing Chen Wenguang Zhao Feng Wang Jie Xu .Dalian National Laboratory for Clean Energy State 《天然气化学杂志》2012,(5):481-487
Preparation of dispersed transition metal oxides catalyst with low oxidation state still remains a challenging task in heterogeneous catalysis.In this study,vanadium oxides supported on zeolite SBA-15 have been prepared under hydrothermal condition using V 2 O 5 and oxalic acid as sources of vanadium and reductant,respectively.The structures of samples,especially the oxidation state of vanadium,and the surface distribution of vanadium oxide species,have been thoroughly characterized using various techniques,including N 2-physisorption,X-ray diffraction(XRD),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS),UV-visible spectra(UV-Vis) and UV-visible-near infrared spectra(UV-Vis-NIR).It is found that the majority of supported vanadium was in the form of vanadium(IV) oxide species with the low valence of vanadium.By adjusting hydrothermal treatment time,the surface distribution of vanadium(IV) oxide species can be tuned from vanadium(IV) oxide cluster to crystallites.These materials have been tested in the hydroxylation of benzene to phenol in liquid-phase with molecular oxygen in the absence of reductant.The catalyst exhibits high selectivity for phenol(61%) at benzene conversion of 4.6%,which is a relatively good result in comparison with other studies employing molecular oxygen as the oxidant. 相似文献
994.
Pollutants release is highly consistent with suspended sediment concentration (SSC) in water column, especially during re-suspension and transport events. The present research focuses on pollutant dynamic release from re-suspended sediment, especially the vertical distribution relationship between them. The sediment erosion experiments on a series of uniform flow are conducted in a circulate flume. Reactive tracer (phosphorus) is used as the contaminant in fine-grained sediments to identify the release characteristic length and time. Experimental results show that the flow condition near-bed depends on the sediment surface roughness. The region with high turbulent intensities corresponds to a high concentration sediment layer. In addition, the SSC decreases with the distance, water depth, and particle grain size. The sediment in a smaller grain size takes much more time to reach equilibrium concentration. Total phosphorus (TP) concentration changes along the water depth as SSC in the initial re-suspension stage, appearing in two obvious concentration regimes: the upper low-concentration layer and the high-concentration near-bottom layer. This layered phenomenon remains for about 3 hours until SSC distri- bution tends to be uniform. Longitudinal desorption plays an important role in long-way transport to reduce the amount of suspended sediment in water column. 相似文献
995.
996.
Straight-chain aliphatic terr-amine N-oxides were found to be useful oxidizing agent for epoxidation of tri- and tetra-substituted perfluoroalkenes under mild conditions in high yields.The process for epoxidation by employing tri-n-butylamine N-oxide gave the best result in the reagent survey. 相似文献
997.
998.
999.
1000.