全文获取类型
收费全文 | 4982篇 |
免费 | 631篇 |
国内免费 | 1541篇 |
专业分类
化学 | 5955篇 |
晶体学 | 118篇 |
力学 | 71篇 |
综合类 | 27篇 |
数学 | 6篇 |
物理学 | 977篇 |
出版年
2024年 | 2篇 |
2023年 | 80篇 |
2022年 | 106篇 |
2021年 | 203篇 |
2020年 | 297篇 |
2019年 | 225篇 |
2018年 | 224篇 |
2017年 | 218篇 |
2016年 | 311篇 |
2015年 | 298篇 |
2014年 | 297篇 |
2013年 | 527篇 |
2012年 | 432篇 |
2011年 | 379篇 |
2010年 | 315篇 |
2009年 | 339篇 |
2008年 | 338篇 |
2007年 | 356篇 |
2006年 | 328篇 |
2005年 | 327篇 |
2004年 | 268篇 |
2003年 | 246篇 |
2002年 | 185篇 |
2001年 | 116篇 |
2000年 | 98篇 |
1999年 | 114篇 |
1998年 | 82篇 |
1997年 | 61篇 |
1996年 | 49篇 |
1995年 | 66篇 |
1994年 | 46篇 |
1993年 | 36篇 |
1992年 | 42篇 |
1991年 | 29篇 |
1990年 | 20篇 |
1989年 | 12篇 |
1988年 | 14篇 |
1987年 | 10篇 |
1986年 | 14篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 2篇 |
排序方式: 共有7154条查询结果,搜索用时 171 毫秒
21.
22.
Sølvi Storsæter 《Surface science》2006,600(10):2051-2063
The heats of adsorption of different C1 and C2 molecules assumed to be present during the initial steps of the Fischer-Tropsch synthesis and activation energies for elementary steps envisioned to occur in the synthesis are calculated for Co by using the unity bond index-quadratic exponential potential (UBI-QEP) method. The preexponential factors for the elementary steps are calculated from transition-state theory, and the rate constants are calculated according to the Arrhenius equation. The activation barrier for hydrogenation of CO is found to be lower compared to hydrogen assisted dissociation of CO, which has a smaller activation barrier than direct dissociation of CO. The reaction steps with high activation barriers are eliminated. Based on this elimination two sets of elementary steps for formation of C1 and C2 alkenes and alkanes in the Fischer-Tropsch synthesis are established: one based on hydrogen assisted CO dissociation (carbide mechanism) and one based on CO hydrogenation (CO insertion mechanism). In addition, one mechanism of producing CO2 from the water-gas shift reaction is proposed. The resulting mechanisms are combined and used in the microkinetic model, which are fitted to experimental results at methanation conditions (T = 483 K or 493 K, p = 1.85 bar and H2/CO = 10) over a Co/Al2O3 Fischer-Tropsch catalyst. A good tuning is obtained by adjusting the C-Co and H-Co binding strengths. The microkinetic modelling based on these assumptions indicates that CO is mainly converted through hydrogenation of CO and that C2 compounds are mainly produced by insertion of CO into a metal-methyl bond. Thus, from the surface coverages and reaction rates predicted by the microkinetic modelling the mechanism can be further reduced to only include the CO insertion mechanism. Hydrogenation of CHO to CH2O is found to be the rate determining initiation step, and insertion of CO into a metal-methyl bond is found to be the rate determining step for chain growth. By using the UBI-QEP method for calculation of activation energies, the activation barriers for dissociation of CO and hydrogenation of surface carbon are found to be too large for the carbide mechanisms to occur. However, experimental data or another theoretical method is necessary in order to support or disprove the calculated activation energies in this work. 相似文献
23.
本文以金属指示剂甲基百里(香)酚蓝作为鳌合剂制得具有相应鳌合基团的负载树脂.研究了该树脂的某些分析特性及其在富集天然水中微量铜、钻、镍、锌中的应用. 相似文献
24.
25.
酞菁钴薄膜的折射率及吸收特性 总被引:3,自引:1,他引:2
通过真空镀法在单晶硅片上制备了酞菁钴薄膜,在波长扫描和入射角可变全自动椭圆偏振光谱仪上研究了CoPc薄膜的椭偏光谱并分析了其电子结构。 相似文献
26.
27.
28.
The solvent extraction of cobalt(II), nickel(II) and copper(II) using 2,4-pent-dione (Hacac) and 4-phenyl-2, 4-but-dione (Hbzac)
is carried out by varying the reagent concentration and pH of the aqueous phase. Each of these metals is quantitatively separated
(≈ 98%) from their binary mixtures with monovalent (Ag), divalent (Mn, Zn, Cd, Hg, Mg, Sn, Pb) and trivalent (Cr, Fe) metals.
The extraction constants are calculated from the metal distribution data using linear regression analysis. The extracted species
is MA2 in each case. A most significant result is separation of copper(II) from iron(III) which otherwise interferes when extracted
from the acidic medium. 相似文献
29.
30.
A simple “chimney” method was used to eliminate the voids in an arrangement of quantum dots sized 2 nm on a solid substrate, which resulted in a large well ordered superlattice of area in the order more than 1 μm2. Based on the principle of speeding up the interparticle interaction of nanoparticles to overcome the particle-substrate one, a lateral centripetal force originated from a glass tube acting as a chimney in a simple evaporation device is imposed. This method allows the packing process to be controlled in a mechanical force field, that is, with the same nanogold dispersion different patterns on a substrate—from separate dots to an ordered compact monolayer or even a multilayer structure—could be easily obtained. 相似文献