全文获取类型
收费全文 | 27050篇 |
免费 | 988篇 |
国内免费 | 157篇 |
专业分类
化学 | 18208篇 |
晶体学 | 255篇 |
力学 | 926篇 |
综合类 | 1篇 |
数学 | 2346篇 |
物理学 | 6459篇 |
出版年
2023年 | 154篇 |
2022年 | 410篇 |
2021年 | 487篇 |
2020年 | 432篇 |
2019年 | 448篇 |
2018年 | 333篇 |
2017年 | 317篇 |
2016年 | 727篇 |
2015年 | 631篇 |
2014年 | 793篇 |
2013年 | 1489篇 |
2012年 | 1947篇 |
2011年 | 2135篇 |
2010年 | 1295篇 |
2009年 | 1143篇 |
2008年 | 1741篇 |
2007年 | 1641篇 |
2006年 | 1532篇 |
2005年 | 1428篇 |
2004年 | 1262篇 |
2003年 | 982篇 |
2002年 | 967篇 |
2001年 | 672篇 |
2000年 | 575篇 |
1999年 | 338篇 |
1998年 | 252篇 |
1997年 | 293篇 |
1996年 | 334篇 |
1995年 | 257篇 |
1994年 | 274篇 |
1993年 | 276篇 |
1992年 | 264篇 |
1991年 | 204篇 |
1990年 | 154篇 |
1989年 | 139篇 |
1988年 | 140篇 |
1987年 | 118篇 |
1986年 | 95篇 |
1985年 | 168篇 |
1984年 | 112篇 |
1983年 | 97篇 |
1982年 | 122篇 |
1981年 | 88篇 |
1980年 | 79篇 |
1978年 | 80篇 |
1977年 | 85篇 |
1976年 | 94篇 |
1975年 | 101篇 |
1974年 | 79篇 |
1973年 | 102篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
994.
995.
The preparation of monolithic poly(butyl methacrylate-co-ethylene dimethacrylate) capillary columns using photoinitiated in situ polymerization within 200 microm i.d. capillaries and their application for microHPLC separations of proteins have been studied. The low resistance to flow characteristic of monolithic columns, enabled the use of very high flow rates of up to 100 microL/min representing a flow velocity of 87 mm/s. Very good separations of a model protein mixture consisting of ribonuclease A, cytochrome c, myoglobin, and ovalbumin was achieved in less than 40 s using a very simple single step gradient of the mobile phase. Interestingly, no effect of the pore size on the separations of proteins was observed for these monolithic columns within the size range of 0.66-2.2 microm. The monolithic microHPLC columns are found very robust and no changes in the long term separation performance and back pressure were observed. 相似文献
996.
[reaction: see text] N-Monosubstituted carbamates are cleanly converted to amides under treatment with trialkylaluminum. This reaction involves an aluminum-assisted internal delivery of alkyl groups. It can be applied to new and mild protecting group strategies for alcohols. 相似文献
997.
Chin-Ping Yang Tzong-Ming Lee 《Journal of polymer science. Part A, Polymer chemistry》1989,27(7):2239-2251
4-Hydroxy-3,5-dibromobenzyl phosphonates (HDBBP) were synthesized by reacting phosphites with 4-hydroxy-3,5-dibromobenzyl bromide (HDBBB) which was prepared in advance by photobromination of 2,6-dibromo-p-cresol (DBPC) produced by bromination of p-cresol. In the reactions between HDBBB and phosphites, the rate of formation of dimethyl 4-hydroxy-3,5-dibromobenzyl phosphonate (HDBBPm) was greater than that of diphenyl 4-hydroxy-3,5-dibromobenzyl phosphonate (HDBBPP); however, quantitative yields for both products were obtainable. The addition of HDBBP as bromine and phosphorus containing flame-retardants to ABS copolymer showed better results than the addition of the same amount of brominated compound, or mixtures of both two compounds. It was also proved that compound containing both bromine and phosphorus exhibited more apparent synergistic effects on flame retardancy than the mixtures of brominated compound and phosphorous containing compound 相似文献
998.
Spatial!y localized electrochemical activity at Al/Al2O3 electrodes has been investigated using scanning electrochemical microscopy (SECM) in order to establish the relationship between localized corrosion of Al (and Al alloys) with the defect structure of the native Al2O3 film. Local electron transfer at microscopic defects (2 to 50 microm radius) was visualized in acetonitrile solutions using the nitrobenzene/nitrobenzene radical anion (Eo approximately -1.6 V vs. Ag/Ag+) and tetracyanoquinodimethane/tetracyanoquinodimethane radical anion couples (Eo approximately -0.3 V) as redox mediators for imaging. SECM investigations revealed no significant differences in electrochemical activity at Al/AI203 electrodes in the two mediator solutions, indicating that electrical conduction at the defect sites is weakly dependent on interfacial potential and the electric field across the Al2O3 film. The density of electroactive defects observed by SECM varied by 2 to 3 orders of magnitude between electrodes prepared from the same source of Al (either 99.450% and 99.9995%) suggesting that electrical conduction in the native oxide is very sensitive to surface preparation. Defect densities as low as approximately 3 sites cm(-2) were readily measured by SECM. 相似文献
999.
Catalysts comprised Pd(OAc)(2) and either PCy(3) or sterically hindered N-heterocyclic carbene ligands provide fast rates for a palladium-catalyzed synthesis of oxindoles by amide alpha-arylation. This catalyst system allowed for room-temperature reactions in some cases and reactions of aryl chlorides at 70 degrees C. Most important, reactions occurred in high yields under mild conditions to form the quaternary carbon in alpha,alpha-disubstituted oxindoles. The combined inter- and intramolecular reaction afforded an efficient synthetic method for formation of alpha-aryloxindole derivatives. Surprisingly, catalysts containing tert-butylphosphine ligands, which have been most reactive for ketone arylations, were less active than those containing PCy(3). Use of new, optically active heterocyclic carbene ligands gave substantial enantioselectivity in formation of an alpha,alpha-disubstituted oxindole. In contrast, a variety of optically active phosphine ligands that were tested gave poor enantioselectivity. Mechanistic studies showed that the reaction involves rate-limiting oxidative addition of aryl halide. Base-induced formation of and reductive elimination from an arylpalladium enolate intermediate were both faster than oxidative addition. Deprotonation of the tethered amide appeared to be faster than reductive elimination of the resulting palladium enolate to form the oxindole product. 相似文献
1000.
Jung HS Lee JK Nastasi M Lee SW Kim JY Park JS Hong KS Shin H 《Langmuir : the ACS journal of surfaces and colloids》2005,21(23):10332-10335
Sol-gel-derived Mg(OH)(2) gel was coated onto TiO(2) nanoparticles, and the subsequent thermal topotactic decomposition of the gel formed a highly nanoporous MgO crystalline coating. The specific surface area of the electrode that was prepared from the core-shell-structured TiO(2) nanoparticles significantly increased compared with that of the uncoated TiO(2) electrode. The increase in the specific surface area of the MgO-coated TiO(2) electrode was attributed to the highly nanoporous MgO coating layer that resulted from the topotactic reaction. Dye adsorption behavior and solar cell performance were significantly enhanced by employing the MgO-coated TiO(2) electrode. Optimized coating of a MgO layer on TiO(2) nanoparticles enhanced the energy conversion efficiency as much as 45% compared to that of the uncoated TiO(2) electrode. This indicates that controlling the extrinsic parameters such as the specific surface area is very important to improve the energy conversion efficiency of TiO(2)-based solar cells. 相似文献