全文获取类型
收费全文 | 4609篇 |
免费 | 613篇 |
国内免费 | 418篇 |
专业分类
化学 | 1205篇 |
晶体学 | 46篇 |
力学 | 1619篇 |
综合类 | 81篇 |
数学 | 1152篇 |
物理学 | 1537篇 |
出版年
2024年 | 13篇 |
2023年 | 33篇 |
2022年 | 80篇 |
2021年 | 118篇 |
2020年 | 142篇 |
2019年 | 99篇 |
2018年 | 125篇 |
2017年 | 154篇 |
2016年 | 202篇 |
2015年 | 137篇 |
2014年 | 238篇 |
2013年 | 374篇 |
2012年 | 259篇 |
2011年 | 250篇 |
2010年 | 201篇 |
2009年 | 261篇 |
2008年 | 273篇 |
2007年 | 284篇 |
2006年 | 289篇 |
2005年 | 270篇 |
2004年 | 213篇 |
2003年 | 210篇 |
2002年 | 150篇 |
2001年 | 120篇 |
2000年 | 135篇 |
1999年 | 146篇 |
1998年 | 100篇 |
1997年 | 119篇 |
1996年 | 100篇 |
1995年 | 87篇 |
1994年 | 71篇 |
1993年 | 41篇 |
1992年 | 55篇 |
1991年 | 57篇 |
1990年 | 36篇 |
1989年 | 21篇 |
1988年 | 39篇 |
1987年 | 27篇 |
1986年 | 20篇 |
1985年 | 23篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 13篇 |
1981年 | 9篇 |
1980年 | 2篇 |
1979年 | 10篇 |
1978年 | 7篇 |
1977年 | 4篇 |
1971年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有5640条查询结果,搜索用时 15 毫秒
81.
Poly(ethylene glycol) (PEG)-supported dendrimers have been synthesized using 2.4,6-trichloro-1,3,5-triazine (TCT) as dendrons and tris(hydroxymethyl)aminomethane as tinkers with high loading capacity, excellent solubility and thermal stability by divergent method. The new synthesized PEG-supported G2.0 dendrimer has 10 times as large functional group loading capacity as commercial PEG3400 with overall yield 44.0%. 相似文献
82.
近年来有不少报道从事于生理活性Drimane类倍半萜的合成,较多采用了分子间Diels-Alder反应来建立Drimane的骨架。本文首次采用内分子Diels-Alder反应作为关键反应合成了Drimane类化合物。同时还在引入了手性诱导基团后,首次完成了从无手性的原料化合物β-紫罗兰酮至光学活性(-)-drimenin1的不对称合成。 相似文献
83.
A qualitative discussion on atom-surface elastic scattering taking place in the presence of laser radiation is given. It is suggested that appreciable effects of laser radiation on diffraction patterns may be expected if the laser radiation is capable of inducing electronic transitions in atoms with a large probability.Research sponsored by the Air Force Office of Scientific Research (AFSC), United States Air Force, under Contract No. F49620-78-C-0005, and the National Science Foundation under Grant No. CHE77-27826. The United States Government is authorized to reproduce and distribute reprints for governmental purposes notwithstanding any copyright notation hereon.Alfred P. Sloan Research Fellow, 1976–1980; Camille and Henry Dreyfus Teacher-Scholar, 1975–1980. 相似文献
84.
Summary Low density polyethylene film is drawn at room temperature four times the original length and subjected to thermal annealing at 60, 80, and 100 °C keeping the film length constant. Long spacing measured by SAXS increased with increasing temperature of annealing; the increase of the long spacing is presumed to be due to the decrease of the number of micelles through relaxation during the annealing. Simultaneous measurement of the changes of the long spacing and the film length by stretching is carried out and stress-extension curves are obtained. The values of the initial moduli of the long spacingE
1 and the film lengthY are very near to each other. Elastic modulus of the crystal latticeE
c is known to be 235 GN/m2 and that of the amorphous regionE
a is found to be 0.15 GN/m2. When higher stress is applied than in the case of the initial modulus, the percentage of extension of film is much greater than that of the long spacing. The discrepancy is explained by the increase of the number of micelles through stress crystallization.Dedicated to Professor Dr. K. Ueberreiter on the occasion of his 70th birthday. 相似文献
85.
Aleš Podgornik 《Journal of separation science》2022,45(12):1984-1996
Modern convection-based supports differ substantially in pore size, porosity, and microstructure topology. Due to such variability, it is challenging to evaluate the contribution of a particular microstructure topology on flow resistance. We demonstrated that the flow resistance parameter ( ) introduced decades ago can be used as a criterion to evaluate the effect of microstructure topology on a pressure drop when the pore size is used as a characteristic support dimension. Furthermore, the value of simple cubic packing was calculated over the entire range of open porosity and compared to the values determined for pressure drop models derived for particular convection-based supports and experimental values of various convection-based supports from the literature. It was shown that different convection-based supports become clustered into distinct groups when plotted according to their and open porosity values, allowing their discrimination. 相似文献
86.
V.M. Jovanovi S. Terzi A.V. Tripkovi K.Dj. Popovi J.D. Lovi 《Electrochemistry communications》2004,6(12):1
Effect of electrochemical oxidation of glassy carbon on deposition of platinum particles and electrocatalytic activity of platinum supported on oxidized glassy carbon (Pt/GCOX) were studied for methanol oxidation in H2SO4 solution. Platinum was potentiostatically deposited from H2SO4 + H2PtCl6 solution. Glassy carbon was anodically polarised in 0.5 M H2SO4 at 2.25 V vs. saturated calomel electrode (SCE) during 35 s. Electrochemical treatment of GC support, affecting not significantly the real Pt surface area, leads to a better distribution of platinum on the substrate and has remarkable effect on the activity. The activity of the Pt/GCOX electrode for methanol oxidation is larger than polycrystalline Pt and for more than one order of magnitude larger than Pt/GC electrode. This increase in activity indicates the pronounced role of organic residues of GC support on the properties of Pt particles deposited on glassy carbon. 相似文献
87.
Arginase isolated from beef liver was covalently attached to a polyacrylamide bead support bearing carboxylic groups activated
by a water-soluble carbodiimide. The most favorable carbodiimide wasN-cyclohexyl-Nt’-(methyl-2-p-nitrophenyl-2-oxoethyl) aminopropyl carbodiimide methyl bromide, but for practical purposes,N-cyclohexyl-Nt’-morpholinoethyl carbodiimide methyl tosylate was used. The optimal conditions for the coupling procedure were
determined. The catalytic activity of the immobilized arginase was 290–340 U/g solid or 2.9–3.4 U/mL wet gel. The pH optimum
for the catalytic activity was pH 9.5, the apparent temperature maximum was at 60°C and Kmapp was calculated to be 0.37M L-arginine. Immobilization markedly improved the conformational stability of arginase. At 60°C,
the pH for maximal stability was found to be 8.0. The immobilized arginase was used for the production of L-ornithine and
D-arginine. 相似文献
88.
Limin Chen Ding Ma Barbara Pietruszka Xinhe Bao 《天然气化学杂志》2006,15(3):181-190
Carbon materials were used as supports for Ag catalysts that are prepared using the conventional wet impregnation method, and their catalytic properties for CO selective oxidation in excess hydrogen at temperatures below 483 K were tested. A variety of techniques, e.g. N2 adsorption, XPS, TPD, UV-Vis DRS, TEM and SEM, were used to determine the influence of physical and chemical properties of the carbon on the properties of Ag catalyst. It was found that defects on the carbon surface served as nucleation sites for silver ions, while functional groups on carbon surface induced their reduction to the metallic form. The formation of silver particles on carbon was governed by homogeneous and/or heterogeneous nucleation during the impregnation and subsequent activation processes. The best catalytic performance was obtained with a Ag/carbon black catalyst with a uniform size distribution of silver nanoparticles (about 12 nm), moderate BET surface area (with a mesoporous structure), and a limited amount of carbon-oxygen groups. The research indicates that carbon materials are potentially good supports for silver catalysts for preferential oxidation of CO in excess hydrogen. 相似文献
89.
A Novel Carbon Nanotube-Supported NiP Amorphous Alloy Catalyst and Its Hydrogenation Activity
下载免费PDF全文
![点击此处可从《天然气化学杂志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Yan Ju Fengyi Li 《天然气化学杂志》2006,15(4):313-318
A carbon nanotube-supported NiP amorphous catalyst (NiP/CNT) was prepared by induced reduction. Benzene hydrogenation was used as a probe reaction for the study of catalytic activity. The effects of the support on the activity and thermal stability of the supported catalyst were discussed based on various characterizations, including XRD, TEM, ICP, XPS, H2-TPD, and DTA. In comparison with the NiP amorphous alloy, the benzene conversion on NiP/CNT catalyst was lower, but the specific activity of NiP/CNT was higher, which is attributed to the dispersion produced by the support, an electron-donating effect, and the hydrogen-storage ability of CNT. The NiP/CNT thermal stability was improved because of the dispersion and electronic effects and the good heat-conduction ability of the CNT support. 相似文献
90.